Poly (meth) acrylic acid (salt)-based particulate water-absorbing agent and production method therefor

ABSTRACT

To provide a disposable diaper enabling reduction in re-wet amount and having an excellent speed of incorporating liquid regardless of concentration and configuration of a water-absorbing agent in an absorbent material. 
     A water-absorbing agent having excellent Gel Capillary Absorption (GCA) and Free Gel Bed Permeability (FGBP) is obtained by crushing a crosslinked hydrogel polymer obtained in a polymerization step to have a specific weight average particle diameter while fluid retention capacity and a surface tension of a water-absorbing agent are adjusted in a specific range, drying the crushed crosslinked hydrogel polymer, and then adding a liquid permeability enhancer thereto during surface crosslinking or after surface crosslinking.

This application is a Divisional Application of U.S. patent applicationSer. No. 15/737,884, filed on Dec. 19, 2017, which was filed under 35USC 371 from PCT/JP2016/068311 on Jun. 20, 2016, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a particulate water-absorbing agentcontaining poly(meth)acrylic acid (salt)-based water-absorbing resinparticles as a main component. More specifically, the present inventionrelates to a particulate water-absorbing agent capable of improving theperformance of an absorbent article such as a disposable diaper.

BACKGROUND ART

A water-absorbing resin (SAP/Super Absorbent Polymer) is a polymergelling agent having water swellable property and water insolubleproperty and exhibits an excellent characteristic for absorbing bodyfluids. For this reason, a water-absorbing agent containing awater-absorbing resin as a main component is often used for absorbentarticles such as disposable diapers and sanitary napkins,agriculture/horticulture water retention agents, industrialwaterproofing materials, and the like. Many kinds of monomers andhydrophilic polymers have been proposed as a raw material of such awater-absorbing resin that constitutes the water-absorbing agent.However, a poly(meth)acrylic acid (salt)-based water-absorbing resinusing (meth)acrylic acid and/or a salt thereof as a main component ismost often used in industry from the viewpoint of price and performance.Such a water-absorbing resin is produced through a polymerization step,a drying step, an optional step for removing a non-dried product, apulverizing step, a classification step, a surface crosslinking step,and the like (Non-Patent Literature 1).

To give a disposable diaper, which is a primary application for awater-absorbing resin (water-absorbing agent), as an example, remediesfor urine leakage and skin rash are in need. As a method for evaluatingthese problems, a method for measuring a re-wet amount from a disposablediaper under pressure and a method for measuring liquid absorption timeof a disposable diaper under pressure have been proposed. It isestimated that, in a case where urine is hardly incorporated into adisposable diaper while the body weight of a wearer of a disposablediaper is applied, or in a case where a water-absorbing agent absorbsurine slowly even if urine is incorporated into the disposable diaper,urine leakage or skin rash occurs. Then, it is considered thatimprovement in absorbability of a water-absorbing agent under pressureand improvement in water absorption speed of the water-absorbing agentlead to reduction in the re-wet amount of a disposable diaper and theliquid absorption time, and further lead to reduction in urine leakageand skin rash.

In recent years, disposable diapers have been widespread and widely usedall over the world. In a disposable diaper, a water-absorbing agent isused in the form of being mixed with pulp. Depending on regions, anabsorbent material having a larger weight of pulp than a weight of awater-absorbing agent, that is, a so-calledlow-water-absorbing-agent-concentration absorbent material(water-absorbing resin concentration: less than 50 wt %) is preferablyused. However, in other regions, an absorbent material having a largerweight of a water-absorbing agent than a weight of pulp, that is, aso-called high-water-absorbing-agent-concentration absorbent material(water-absorbing resin concentration: 50 wt % or more) is preferablyused, and an absorbent material containing only a water-absorbing resinwithout use of pulp is also preferably used.

Conventionally, in order to reduce the re-wet amount of these disposablediapers and to reduce the liquid absorption time of these disposablediapers, many techniques for improving absorption characteristics underpressure have been proposed.

Specific examples of the proposition include a technique for using awater-absorbing agent having a large sum of fluid retention capacitiesunder four different of pressures (PAI) for a disposable diaper (PatentLiterature 1), a technique for improving diffusivity of liquid not onlyin a vertical direction but also in a horizontal direction in an SAPlayer under pressure (Patent Literatures 2 and 3), a technique forimproving fluid retention capacity under pressure while the SAP amountper unit area is large (Patent Literatures 4 and 5), and a technique forimproving fluid retention capacity under pressure measured while thereis a difference in height between a glass filter in contact with awater-absorbing agent and a liquid surface of a liquid supply side(Patent Literatures 6 and 7). Particularly, in Patent Literature 7, itis proposed that a new parameter for evaluating liquid suctioncapability in a short time that is called Gel Capillary Absorption (GCA)is introduced and the re-wet amount can be reduced as the value of GCAis increased.

Indeed, in the case of using a water-absorbing agent having improvedGCA, in the absorbent material, which has been hitherto mainly used,containing a water-absorbing agent in a small amount, the effect ofreducing the re-wet amount is exerted; on the other hand, a problemarises in that in a high-concentration absorbent material or anabsorbent material without use of pulp, the expected effect is notalways able to be recognized in view of the speed of incorporatingliquid and the re-wet amount.

Further, as a method for producing a water-absorbing agent havingimproved GCA in Patent Literature 7, a technique of introducing foaming,a technique of mixing a fine particulate dried product of awater-absorbing resin with water or hot water at a high speed so as tobe granulated, and the like have been used.

Further, as a technique for crushing a polymerization gel, there havebeen proposed a technique for crushing a polymerization gel with aspecific energy to form gel particles (Patent Literatures 8 to 10) and atechnique for crushing a polymerization gel with a gel-crusher having aspecific shape (a meat chopper) (Patent Literatures 11 to 14).

In a foaming technique described in the above-described PatentLiterature 7 and the like, dispersion stabilization of bubbles in amonomer solution is difficult to achieve, and stable production isdifficult. Further, in a case where bubbles are stabilized by a largeamount of a surfactant, a problem such as a decrease in surface tensionmay occur.

As for the technique of introducing foaming described in theabove-described Patent Literature 7 and the like, there are problems inthat the amount of fine powder, which does not become a product,produced is increased during crushing so that production efficiency islargely decreased, and in that physical properties such as fluidretention capacity under pressure are largely degraded since a driedproduct of the water-absorbing resin becomes brittle so as to be easilydamaged in a transporting step. Further, as for a granulating techniquedescribed in the above-described Patent Literature 7 and the like, thereis a room for improvement in production efficiency since water or hotwater is added to the fine particulate dried product of thewater-absorbing resin and then the mixture is granulated and dried, andthere is a problem in performance degradation due to deterioration ofthe water-absorbing resin caused by performing a drying process multipletimes. Furthermore, in techniques relating to a gel-crusher and crushingconditions (Patent Literatures 8 to 14) and techniques of adding anadditive such as polyethylene glycol or a hydrophobic substance duringpolymerization of a polymerization gel or during crushing of apolymerization gel (Patent Literatures 15 to 12), there are problems inthat the crushing conditions are not sufficient for improving GCA and inthat a surface tension is largely decreased. Techniques of producinggranulated particles by a reverse phase suspension polymerization method(Patent Literatures 21 and 22) have also been proposed. However, thereare problems in that processes are cumbersome and in that troubles dueto remaining of an organic solvent occur. Moreover, the effect ofimproving the performance of a disposable diaper is not sufficient, forexample, the re-wet amount is increased by a decrease in surfacetension.

CITATION LIST Patent Literatures

Patent Literature 1: U.S. Pat. No. 5,601,542

Patent Literature 2: U.S. Pat. No. 5,760,080

Patent Literature 3: U.S. Pat. No. 5,797,893

Patent Literature 4: U.S. Pat. No. 6,297,335

Patent Literature 5: WO 2011/040472 A

Patent Literature 6: U.S. Pat. No. 7,108,916

Patent Literature 7: WO 2015/129917 A

Patent Literature 8: WO 2011/126079 A

Patent Literature 9: WO 2013/002387 A

Patent Literature 10: WO 2014/118024 A

Patent Literature 11: WO 2015/030129 A

Patent Literature 12: WO 2015/030130 A

Patent Literature 13: EP 0574248 A

Patent Literature 14: U.S. Pat. No. 5,275,773

Patent Literature 15: WO 2008/096713 A

Patent Literature 16: WO 2009/075204 A

Patent Literature 17: JP 4341143 B1

Patent Literature 18: WO 2010/073658 A

Patent Literature 19: WO 97/19116 A

Patent Literature 20: WO 2004/096304 A

Patent Literature 21: U.S. Pat. No. 5,180,798

Patent Literature 22: US 2013/0,130,017 A

Non-Patent Literature

Non-Patent Literature 1: Modern Superabsorbent Polymer Technology (1998)

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to solve at least one of problemsdescribed below.

An object of the present invention is to provide a poly(meth)acrylicacid (salt)-based particulate water-absorbing agent enabling reductionin re-wet amount and having an excellent speed of incorporating liquid.

Further, another object of the present invention is to provide a methodfor producing a poly(meth)acrylic acid (salt)-based particulatewater-absorbing agent enabling reduction in re-wet amount and having anexcellent speed of incorporating liquid.

Still another object of the present invention is to provide a hygienicmaterial (for example, a disposable diaper) enabling reduction in re-wetamount in a high concentration of an absorbing agent and having anexcellent speed of incorporating liquid.

Solution to Problem

Hereinbefore, many water-absorbing resins whose parameters arecontrolled have been proposed, and the present inventors have filed thepatent application shown in Patent Literature 7, which is focused on GelCapillary Absorption (GCA) that is a new parameter with respect to therelated arts such as Patent Literatures 1 to 6. However, insufficientpoints have still been found.

In this regard, in order to solve the above problem, the presentinventors have conducted intensive studies. As a result, liquidpermeability has still been insufficient in Patent Literature 7 which isfocused on Gel Capillary Absorption (GCA) that is a new parameter. Thepresent inventors have found that the above-described problem can besolved when Free Gel Bed Permeability (FGBP) that is a new index ofliquid permeability is high in addition to GCA.

That is, the present invention provides a method for producing apoly(meth)acrylic acid (salt)-based particulate water-absorbing agentcontaining poly(meth)acrylic acid (salt)-based water-absorbing resinparticles as a main component, the method including: (i) a step forpreparing a (meth)acrylic acid (salt)-based aqueous monomer solution;(ii) a step for polymerizing the (meth)acrylic acid (salt)-based aqueousmonomer solution; (iii) a step for gel-crushing a crosslinked hydrogelpolymer during polymerization or after polymerization to obtain hydrogelparticles;

(iv) a step for drying the hydrogel particles to obtain a dried product;(v) a step for pulverizing and/or classifying the dried product toobtain water-absorbing resin powder; (vi) a step for surfacecrosslinking the water-absorbing resin powder to obtain water-absorbingresin particles; and (vii) a step for adding a liquid permeabilityenhancer to the water-absorbing resin powder or the water-absorbingresin particles, wherein the method further includes adding an adhesioncontrolling agent, which controls adhesion of the crosslinked hydrogelpolymer and/or the hydrogel particles, in the step (iii) or before thestep (iii), a solids content of the hydrogel particles is adjusted to 10wt % to 80 wt % and a weight average particle diameter of the hydrogelparticles converted to the dried product is adjusted to 50 μm to 650 μm,and a surface tension of the poly(meth)acrylic acid (salt)-basedparticulate water-absorbing agent is adjusted to 60 mN/m or more, and afluid retention capacity without pressure (CRC) is adjusted to 28 g/g ormore.

Further, according to the present invention, there is provided apolyacrylic acid (salt)-based particulate water-absorbing agentincluding polyacrylic acid (salt)-based water-absorbing resin particlesas a main component, the polyacrylic acid (salt)-based particulatewater-absorbing agent satisfying the following (1) to (5): (1) a fluidretention capacity without pressure (CRC) is 28 g/g or more; (2) GCA is28.0 g/g or more; (3) a relation between FGBP and GCA satisfies, in acase where GCA is in a range of 28.0 g/g or more and less than 36.0 g/g,FGBP≥−10×10⁻⁹×GCA+380×10⁻⁹ cm², and in a case where GCA is 36.0 g/g ormore, FGBP≥30×10⁻⁹ cm²; (4) a weight average particle diameter (D50) ofthe particulate water-absorbing agent is 300 μm to 500 μm; and (5) asurface tension is 60 mN/m or more.

Effect of the Invention

According to the present invention, at least one of the followingeffects is achieved.

It is possible to provide a poly(meth)acrylic acid (salt)-basedparticulate water-absorbing agent enabling reduction in re-wet amountand having an excellent speed of incorporating liquid.

Further, it is possible to provide a method for producing apoly(meth)acrylic acid (salt)-based particulate water-absorbing agentenabling reduction in re-wet amount and having an excellent speed ofincorporating liquid.

It is possible to provide a hygienic material (for example, a disposablediaper) enabling reduction in re-wet amount in a high concentration ofan absorbing agent and having an excellent speed of incorporatingliquid.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an apparatus used for GCA measurementof the present invention.

FIG. 2 is an SEM photograph (magnification: 30) of water-absorbing resinpowder of Example 9.

FIG. 3 is an SEM photograph (magnification: 130) of the water-absorbingresin powder of Example 9.

FIG. 4 is an SEM photograph (magnification: 30) of water-absorbing resinpowder of Comparative Example 1.

FIG. 5 is an SEM photograph (magnification: 130) of water-absorbingresin powder of Comparative Example 5.

FIG. 6 is a correlation diagram showing a relation between Examples andComparative Examples.

FIG. 7 is a schematic diagram of an apparatus used for evaluating anabsorbent material of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a particulate water-absorbing agent according to thepresent invention and a production method therefor is described indetail. However, the scope of the present invention is not limitedthereto but can be appropriately embodied with modifications other thanthe following exemplary embodiments but not departing from the gist ofthe present invention. Specifically, the present invention is notlimited to the following embodiments, but can be modified variously inthe range indicated by claims. Embodiments obtained by appropriatelycombining technical means disclosed in different embodiments are alsoincluded in the technical scope of the present invention.

[1] Definition of Terms

(1-1) “Particulate Water-Absorbing Agent”

In the present invention, the term “particulate water-absorbing agent”is a gelling agent of water-based liquid containing water-absorbingresin particles as a main component (preferably 60 wt % or more, morepreferably 80 wt % or more, and most preferably 90 wt % or more). Asanother optional component, the particulate water-absorbing agent maycontain water, inorganic fine particles, a moisture absorption blockinginhibitor, a cationic polymer compound, a water-soluble polyvalent metalcation-containing compound, a surfactant, a dust inhibitor, a coloringpreventing agent, a urine resistance improver, a deodorant, a perfume,an antimicrobial agent, a foaming agent, a pigment, a dye, a fertilizer,an oxidizing agent, a reducing agent, and the like in an amount of 0 wt% to 10 wt %, preferably 0.1 wt % to 1 wt % for each. Incidentally, the“particulate water-absorbing agent” is simply referred to as the“water-absorbing agent” in some cases.

(1-2) “Water-Absorbing Resin”

In the present invention, a “water-absorbing resin” means a polymergelling agent having water swellable property and water insolubleproperty. Incidentally, “water swellable” indicates that CRC (fluidretention capacity without pressure) as defined in ERT441.2-02 is 5 g/gor more, and “water insoluble” indicates that Ext (soluble component) asdefined in ERT470.2-02 is 0 wt % to 50 wt %.

Further, the whole amount (100 wt %) of the water-absorbing resin is notnecessarily consisting of a polymer. The water-absorbing resin maycontain an additive or the like in a range maintaining theabove-mentioned properties. In the present invention, a water-absorbingresin composition containing a small amount of an additive is alsocollectively referred to as a water-absorbing resin. Incidentally, thewater-absorbing resin preferably has a powdery shape, and particularlypreferably has a powdery shape having a particle size described below.Incidentally, in the present invention, the “water-absorbing resin” isalso referred to as “water-absorbing resin powder” or “water-absorbingresin particles” in some cases.

(1-3) “Poly(Meth)Acrylic Acid (Salt)-Based Water-Absorbing Resin”

The term “poly(meth)acrylic acid (salt)-based water-absorbing resin” inthe present invention means a polymer optionally containing a graftcomponent and mainly containing a (meth)acrylic acid and/or a saltthereof (hereinafter, referred to as a (meth)acrylic acid (salt)) as arepeating unit.

Specifically, the poly(meth)acrylic acid (salt)-based water-absorbingresin means a polymer containing a (meth)acrylic acid (salt) in anamount of 50 mol % to 100 mol %, and a water-absorbing resin containinga (meth)acrylic acid (salt) in an amount of preferably 70 mol % to 100mol %, more preferably 90 mol % to 100 mol %, and particularlypreferably substantially 100 mol % per the total monomers used inpolymerization (excluding a crosslinking agent). Further, in the presentinvention, a polymer having a poly(meth)acrylic acid salt type(neutralized) is also collectively referred to as a poly(meth)acrylicacid (salt)-based water-absorbing resin.

(1-4) “EDANA” and “ERT”

“EDANA” is an abbreviation for European Disposables and NonwovensAssociations. “ERT” is an abbreviation of methods for measuring awater-absorbing resin (EDANA Recommended Test Methods), which is anEuropean standard (essentially the world standard). In the meantime,inthe present invention, unless otherwise specified, physical propertiesof a water-absorbing resin are measured in conformity with the originaldocument of ERT (publicly-known document: revised in 2002).

(1-4-1) “CRC” (ERT441.2-02)

“CRC” is an abbreviation for Centrifuge Retention Capacity, and meansfluid retention capacity without pressure (also referred to as “fluidretention capacity” in some cases) of a water-absorbing resin.Specifically, “CRC” means fluid retention capacity (unit; g/g) obtainedafter 0.2 g of a water-absorbing resin is input in a bag made ofnonwoven fabric, is then freely swollen in 0.9 wt % of aqueous sodiumchloride solution in a largely excessive amount for 30 minutes and thenis further drained by a centrifuge (250 G).

(1-4-2) “AAP” (ERT442.2-02)

“AAP” is an abbreviation of Absorption Against

Pressure and means fluid retention capacity under pressure of awater-absorbing resin. Specifically, “AAP” means fluid retentioncapacity (unit; g/g) obtained after 0.9 g of a water-absorbing resin isswollen in 0.9 wt % of aqueous sodium chloride solution in a largelyexcessive amount under a load of 2.06 kPa (21 g/cm², 0.3 psi) for 1hour. Further, Absorption Against Pressure is expressed as AbsorptionUnder Pressure in ERT442.2-02, but these are substantially the same.

(1-4-3) “PSD” (ERT420.2-02)

“PSD” is an abbreviation of Particle Size Distribution and means aparticle size distribution of a water-absorbing resin measured by sieveclassification. Incidentally, the weight average particle diameter (D50)and the logarithmic standard deviation (σζ) in particle sizedistribution are measured by the same method as “(3) Mass-AverageParticle Diameter (D50) and Logarithmic Standard Deviation (σζ) ofParticle Diameter Distribution” described in U.S. Pat. No. 7,638,570.

(1-4-4) “Ext” (ERT470.2-02)

“Ext” is an abbreviation of Extractables and means a water solublecomponent (amount of water-soluble component) of a water-absorbingresin. Specifically, “Ext” is the amount of dissolved polymer (unit; wt%) after 1.0 g of a water-absorbing resin is added to 200 ml of 0.9 wt %of aqueous sodium chloride solution and the resulting mixture is stirredfor 16 hours at 500 rpm. The amount of dissolved polymer is measured bypH titration.

(1-5) GCA (Refer to Patent Literature 7)

“GCA” is a new parameter (Gel Capillary Absorption) firstly focused anddescribed in Patent Literature 7 and evaluates liquid absorption abilityunder a load of 0.05 psi for 10 minutes with a difference in height of10 cm between the upper surface of a glass filter and the meniscus atthe lower portion of a Mariotte tube.

(1-6) FGBP (Free Gel Bed Permeability)

FGBP measures liquid permeability of a gel layer under load after 0.9 gof a water-absorbing agent is freely swollen in a cell without anychanges by a physiological saline solution. While GBP known as therelated prior art that is described as “GBP” in the patent literature WO2004/096304 A or the like is measurement for a specific particles size(only particles having a particle size of 300 μm to 600 μm are sieved)in the whole particles, FGBP is measurement for a particle size withoutany changes and without sieving (that is, whole absorbing agentparticles). The original liquid permeability of the water-absorbingagent can be evaluated with use of FGBP.

(1-7) Others

In the present specification, “X to Y” indicating a range means “equalto or more than X and equal to or less than Y.” Further, “t (ton)” as aunit of weight means “etricton.” Moreover, unless otherwise specified,“ppm” means “ppm by weight.” Furthermore, “weight” and “mass,” “wt %”and “mass %,” and “ parts by weight” and “parts by mass” are assumed tobe synonymous, respectively. In addition, “ . . . acid (salt)” means “ .. . acid and/or a salt thereof” and “(meth)acrylic” means “acrylicand/or methacrylic.” Furthermore, unless otherwise specified, physicalproperties and the like are measured at room temperature (20° C. to 25°C.) at a relative humidity of 40% RH to 50% RH.

[2] Method for Producing Particulate Water-Absorbing Agent

As described above, a method for producing a particulate water-absorbingagent according to the present invention is a method for producing apoly(meth)acrylic acid (salt)-based particulate water-absorbing agentcontaining poly(meth)acrylic acid (salt)-based water-absorbing resinparticles as a main component, the method including: (i) a step forpreparing a (meth)acrylic acid (salt)-based aqueous monomer solution;(ii) a step for polymerizing the (meth)acrylic acid (salt)-based aqueousmonomer solution; (iii) a step for gel-crushing a crosslinked hydrogelpolymer during polymerization or after polymerization to obtain hydrogelparticles; (iv) a step for drying the hydrogel particles to obtain adried product; (v) a step for pulverizing and/or classifying the driedproduct to obtain water-absorbing resin powder; (vi) a step for surfacecrosslinking the water-absorbing resin powder to obtain water-absorbingresin particles; and (vii) a step for adding a liquid permeabilityenhancer to the water-absorbing resin powder or the water-absorbingresin particles, in which the method further includes adding an adhesioncontrolling agent, which controls adhesion of the crosslinked hydrogelpolymer and/or the hydrogel particles, in the step (iii) or before thestep (iii), a solids content of the hydrogel particles is adjusted to 10wt % to 80 wt % and a weight average particle diameter of the hydrogelparticles converted to the dried product is adjusted to 50 μm to 650 μm,a surface tension of the poly(meth)acrylic acid (salt)-based particulatewater-absorbing agent is adjusted to 60 mN/m or more, and a fluidretention capacity without pressure (CRC) is adjusted to 28 g/g or more.

As described above, by crushing the crosslinked hydrogel polymerobtained in a polymerization step to have a specific weight averageparticle diameter (a weight average particle diameter of the hydrogelparticles converted to the dried product of 50 μm to 650 μm) and dryingthe crushed crosslinked hydrogel polymer, and then adding a liquidpermeability enhancer thereto during surface crosslinking or aftersurface crosslinking, it is possible to obtain a water-absorbing agenthaving excellent Gel Capillary Absorption (GCA) and Free Gel BedPermeability (FGBP) and to achieve the desired object of the presentinvention.

Further, according to the present invention, there is also provided amethod for producing a poly(meth)acrylic acid (salt)-based particulatewater-absorbing agent containing poly(meth)acrylic acid (salt)-basedwater-absorbing resin particles as a main component, the methodincluding: (i) a step for preparing a (meth)acrylic acid (salt)-basedaqueous monomer solution; (ii) a step for polymerizing the (meth)acrylicacid (salt)-based aqueous monomer solution; (iii) a step forgel-crushing a crosslinked hydrogel polymer during polymerization orafter polymerization to obtain hydrogel particles; (iv) a step fordrying the hydrogel particles to obtain a dried product; (v) a step forpulverizing and/or classifying the dried product to obtainwater-absorbing resin powder; (vi) a step for surface crosslinking thewater-absorbing resin powder to obtain water-absorbing resin particles;and (vii) a step for adding a liquid permeability enhancer to thewater-absorbing resin powder or the water-absorbing resin particles, inwhich the method further includes adding an adhesion controlling agent,which controls adhesion of the crosslinked hydrogel polymer and/or thehydrogel particles, in the step (iii) or before the step (iii), a solidscontent of the hydrogel particles is adjusted to 10 wt % to 80 wt % anda weight average particle diameter of the hydrogel particles is adjustedto 100 μm to 900 μm, a surface tension of the poly(meth)acrylic acid(salt)-based particulate water-absorbing agent is adjusted to 60 mN/m ormore, and a fluid retention capacity without pressure (CRC) is adjustedto 28 g/g or more.

As described above, by crushing the crosslinked hydrogel polymerobtained in a polymerization step to have a specific weight averageparticle diameter (a weight average particle diameter of the hydrogelparticles of 100 μm to 900 μm) and drying the crushed crosslinkedhydrogel polymer, and then adding a liquid permeability enhancer theretoduring surface crosslinking or after surface crosslinking, it ispossible to obtain a water-absorbing agent having excellent GelCapillary Absorption (GCA) and Free Gel Bed Permeability (FGBP) and toachieve the desired object of the present invention.

In the present specification, at least one of a case where “the weightaverage particle diameter of the hydrogel particles converted to thedried product is 50 μm to 650 μm” and a case where “the weight averageparticle diameter of the hydrogel particles is 100 μm to 900 μm” issimply referred to as a case where “the particle diameter aftergel-crushing is significantly small” in some cases.

Incidentally, in the above-described production method, it is preferableto “sequentially” perform the step (i) to step (vii) in this order, buteach step may be simultaneously performed with the prior step or thepost step.

The period of time between the above steps including transportation timeor storage time is appropriately determined, and is preferably 0 secondsor longer and 2 hours or shorter, and more preferably 1 second or longerand 1 hour or shorter.

Hereinafter, the method for producing a particulate water-absorbingagent according to the present invention is described mainly in atemporal order. However, each of the production methods is only requiredto include the above-mentioned essential steps, and may further includeanother step within a range not departing from the gist of each of theproduction methods.

(2-1) Step for Preparing (Meth)Acrylic Acid (Salt)-Based Aqueous MonomerSolution (Step (i))

In the present specification, “(meth)acrylic acid (salt)-based aqueousmonomer solution” is an aqueous monomer solution containing a(meth)acrylic acid (salt) as a main component, and optionally containinga crosslinking agent, a graft component, or a minor component (chelatingagent, surfactant, dispersant, or the like) and prepared containingthese components constituting a water-absorbing resin. In other words,“(meth)acrylic acid (salt)-based aqueous monomer solution” means asolution subjected to polymerization as it is after being added with apolymerization initiator.

The above-mentioned (meth)acrylic acid (salt) may be unneutralized or asalt type (fully neutralized or partially neutralized). Further, theaqueous monomer solution may have a concentration larger than asaturation concentration. Even a supersaturated aqueous solution of a(meth)acrylic acid (salt) or a slurry aqueous solution thereof (aqueousdispersion) is assumed to be the (meth)acrylic acid (salt)-based aqueousmonomer solution of the present invention. Incidentally, from theviewpoint of physical properties of the resulting water-absorbing resin,it is preferable to use a (meth)acrylic acid (salt)-based aqueousmonomer solution having a concentration equal to or lower than thesaturation concentration.

Further, as a solvent for dissolving a monomer, water is preferable. A(meth)acrylic acid (salt)-based monomer is handled as an aqueoussolution. Here, “aqueous solution” is not limited to a case where 100 wt% of the solvent is water. A water-soluble organic solvent (for example,alcohol or the like) may be used together with water in an amount of 0wt % to 30 wt %, preferably 0 wt % to 5 wt % per 100 wt % of the totalamount of the solvent.

In the present invention, these solutions are assumed to be aqueoussolutions.

In the present specification, “(meth)acrylic acid (salt)-based aqueousmonomer solution during preparation” described below means an aqueoussolution of the above-mentioned (meth)acrylic acid (salt) before all thecomponents are mixed with the aqueous monomer solution containing the(meth)acrylic acid (salt) as a main component. Specifically, an aqueous(meth)acrylic acid solution or a fully neutralized or partiallyneutralized aqueous (meth)acrylic acid (salt) solution correspondsthereto.

By further neutralizing the (meth)acrylic acid (salt)-based aqueousmonomer solution during preparation or mixing water as a solvent or theabove minor component or the like to the (meth)acrylic acid (salt)-basedaqueous monomer solution, a final (meth)acrylic acid (salt)-basedaqueous monomer solution is obtained. Incidentally, this final(meth)acrylic acid (salt)-based aqueous monomer solution before beingput into a polymerization apparatus or before starting to polymerizeafter being put into the polymerization apparatus is referred to as“(meth)acrylic acid (salt)-based aqueous monomer solution afterpreparation before a polymerization step.”

(2-1-1) Monomer

For the water-absorbing resin of the present invention, a monomercontaining a (meth)acrylic acid (salt) as a main component is used. The“main component” means that a (meth)acrylic acid (salt) is contained inan amount of usually 50 mol % or more, preferably 70 mol % or more, morepreferably 80 mol % or more, still more preferably 90 mol % or more, andparticularly preferably 95 mol % or more (the upper limit is 100 mol %)per the total amount of monomers (excluding an internal crosslinkingagent).

Incidentally, in the present invention, the poly(meth)acrylic acid(salt) is not limited to an unneutralized poly(meth)acrylic acid (salt)(the rate of neutralization is 0 mol %), but encompasses a partiallyneutralized or fully neutralized (the rate of neutralization is 100 mol%) poly(meth)acrylic acid (salt).

As long as a (meth)acrylic acid (salt) is contained as a main component,a monomer to become a water-absorbing resin by polymerization may becontained in addition thereto. Examples thereof include an anionicunsaturated monomer (salt) such as (anhydrous) maleic acid, itaconicacid, cinnamic acid, vinyl sulfonic acid, allyl toluene sulfonic acid,vinyl toluene sulfonic acid, styrene sulfonic acid,2-(meth)acrylamide-2-methyl propanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloyl propane sulfonic acid, or2-hydroxyethyl (meth)acryloyl phosphate and the like; a mercaptogroup-containing unsaturated monomer; a phenolic hydroxygroup-containing unsaturated monomer; an amide group-containingunsaturated monomer such as (meth)acrylamide, N-ethyl (meth)acrylamide,or N,N-dimethyl (meth)acrylamide and the like; and an aminogroup-containing unsaturated monomer such as N,N-dimethyl aminoethyl(meth) acrylate, N,N-dimethyl aminopropyl (meth)acrylate, orN,N-dimethyl aminopropyl (meth)acrylamide and the like. Further, thewater-absorbing resin may contain the above-mentioned other monomers asa copolymer component.

In the present invention, the rate of neutralization of a (meth)acrylicacid (salt)-based monomer or a crosslinked hydrogel polymer afterpolymerization is not particularly limited, but is preferably 40 mol %to 90 mol %, more preferably 50 mol % to 85 mol %, and still morepreferably 65 mol % to 80 mol % from the viewpoint of physicalproperties of the resulting water-absorbing resin or a reactivity of asurface crosslinking agent. In this regard, according to the preferredembodiment of the present invention, the rate of neutralization of theacrylic acid (salt)-based aqueous monomer solution in the step (i) is 40mol % to 90 mol %.

However when the rate of neutralization is low, the water absorptionspeed tends to be lowered (for example, the water absorption time byVortex method is increased). On the contrary, when the rate ofneutralization is high, the reactivity between the poly(meth)acrylicacid (salt)-based water-absorbing resin and the surface crosslinkingagent (particularly, a dehydration-reactive surface crosslinking agentdescribed below) tends to be lowered, the productivity tends to bereduced, or the fluid retention capacity under pressure (for example,AAP) tends to be reduced. Therefore, the rate of neutralization ispreferably in the above-mentioned range.

The neutralization may be conducted to a hydrogel (crosslinked hydrogelpolymer) after polymerization, as well as to the monomer and/or theaqueous monomer solution before polymerization, and both of these may beadopted together. When neutralization is performed multiple times, it ispreferable that, by taking the addition amounts of all the basiccompounds in consideration, the rate of neutralization is adjusted inthe above-mentioned range.

In particular, when unneutralized polymerization in which the rate ofneutralization is 0 mol % is conducted and neutralization is conductedin the subsequent step, the neutralization does not become uniform, andununiform extent of surface treatment for each of the water-absorbingresin particles occurs in the surface treatment step. Thus, there is aconcern that the fluid retention capacity under pressure is largelyreduced. In this regard, it is preferable to perform neutralizationpolymerization.

Further, from the viewpoint of the fluid retention capacity withoutpressure (CRC) or the water absorption speed of the resultingwater-absorbing resin particles (particulate water-absorbing agent), apart or the whole of the (meth)acrylic acid (salt)-based monomer or thecrosslinked hydrogel polymer may be a salt type. One or more kinds ofmonovalent salt such as a sodium salt, a lithium salt, a potassium salt,an ammonium salt, or an amine are preferable. Among these, one or morekinds of alkali metal salt are more preferable, and a sodium salt and/ora potassium salt are still more preferable. A sodium salt isparticularly preferable from the viewpoint of cost and physicalproperties.

(2-1-2) Internal Crosslinking Agent

In the present invention, in the above-mentioned polymerization, aninternal crosslinking agent is used, as required. As the internalcrosslinking agent, a known internal crosslinking agent can be used.Examples thereof include N,N′-methylene bis(meth)acrylamide,(poly)ethylene glycol di(meth)acrylate, (poly)propylene glycoldi(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerintri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxidemodified trimethylolpropane tri(meth)acrylate, pentaerythritolhexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallylphosphate, triallylamine, poly(meth)allyloxy alkane, (poly)ethyleneglycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol,polyethylene glycol, propylene glycol, glycerin, 1,4-butanediol,pentaerythritol, ethylenediamine, ethylene carbonate, propylenecarbonate, polyethylene imine, and glycidyl (meth)acrylate and the like.One or two or more kinds of these compounds can be used in considerationof the reactivity. Among these, it is preferable to use a compoundhaving two or more polymerizable unsaturated groups.

The amount of the internal crosslinking agent used can be determinedappropriately depending on the desired physical properties of thewater-absorbing resin, but is appropriately adjusted such that CRC ofthe water-absorbing agent of the present invention is 28 g/g or more.

Specifically, the amount of the internal crosslinking agent used ispreferably 0.001 mol % to 1 mol %, more preferably 0.005 mol % to 0.5mol %, still more preferably 0.01 mol % to 0.3 mol %, and particularlypreferably 0.01 mol % to 0.1 mol % with respect to 100 mol % of the(meth)acrylic acid (salt)-based monomer.

When the amount to use thereof is less than 0.001 mol %, a solublecontent of the resulting water-absorbing resin becomes large, the waterabsorption amount under pressure cannot be sufficiently secured, andvalues of GCA and FGBP also not an appropriate value, which is notpreferable. On the other hand, when the amount to use thereof is morethan 1 mol %, the crosslink density of the resulting water-absorbingresin becomes too high and the water absorption amount cannot besufficiently secured, and as a result, the value of GCA becomes low,which is not preferable.

In order to introduce a crosslinking structure into a polymer using theinternal crosslinking agent, the internal crosslinking agent is onlyrequired to be added to a reaction system before, during, or afterpolymerization, or after neutralization of the monomer. Incidentally,the internal crosslinking agent may be added to a reaction system atonce or in a divided manner.

(Adhesion Controlling Agent)

In order to sophisticatedly solve the problem of the present invention,an adhesion controlling agent (can also be called a fusion controllingagent) specifically described in (2-3-2) may be added during or afterthe step for preparing the (meth)acrylic acid (salt)-based aqueousmonomer solution, and specifically, specifically, is added in the step(iii) or before the step (iii).

(2-2) Polymerization Step (Step (ii))

Examples of a polymerization method for obtaining the water-absorbingresin particles (crosslinked hydrogel polymer) of the present inventionmay include spray polymerization, droplet polymerization, bulkpolymerization, precipitation polymerization, aqueous solutionpolymerization, and reverse phase suspension polymerization. In order tosolve the problem of the present invention, the aqueous solutionpolymerization or the reverse phase suspension polymerization, whichuses an aqueous solution of a monomer, is preferable.

Incidentally, in the aqueous solution polymerization, an aqueous monomersolution is polymerized without using a dispersion solvent, and themethod is disclosed, for example, in U.S. Pat. Nos. 4,625,001,4,873,299, 4,286,082, 4,973,632, 4,985,518, 5,124,416 5,250,640,5,264,495, 5,145,906, 5,380,808, EP 0811636 B, EP 0955086 B, EP 0922717B, and the like.

Further, in the reverse phase suspension polymerization, an aqueousmonomer solution is polymerized by being suspended in a hydrophobicorganic solvent, and the method is disclosed, for example, in U.S Pat.Nos. 4,093,776, 4,367,323, 4,446,261, 4,683,274, 5,244,735, and thelike. A monomer, a polymerization initiator, and the like disclosed inthese Patent Literatures can be also applied to the present invention.The concentration of an aqueous monomer solution during polymerizationis not particularly limited, but is preferably 20 wt % to the saturationconcentration or less, more preferably 25 wt % to 80 wt %, and stillmore preferably 30 wt % to 70 wt %. When the concentration is 20 wt % ormore, high productivity can be achieved, which is preferable. In themeantime, in polymerization of a monomer in the state of a slurry(aqueous dispersion of (meth)acrylate), physical properties are lowered,and therefore polymerization is preferably performed with aconcentration equal to or lower than the saturation concentration (referto JP 1-318021 A).

The polymerization step in the present invention can be performed at anormal pressure, a reduced pressure, or an increased pressure, but ispreferably performed at a normal pressure (or in the vicinity thereof,usually ±10 mmHg). Further, in order to improve the physical propertiesby accelerating polymerization, during polymerization, a step fordegassing dissolved oxygen (for example, a step for replacement with aninert gas) may be provided, as required. Further, the temperature at thetime of initiating polymerization depends on the kind of apolymerization initiator used, and is preferably 15° C. to 130° C. andmore preferably 20° C. to 120° C.

(Polymerization Initiator)

The polymerization initiator used in the present invention is determinedappropriately depending on a polymerization form, and is notparticularly limited. Examples thereof include a photodecomposition typepolymerization initiator, a thermal decomposition type polymerizationinitiator, and a redox type polymerization initiator. Thesepolymerization initiators initiate polymerization of the presentinvention.

Examples of the photodecomposition type polymerization initiator includea benzoin derivative, a benzyl derivative, an acetophenone derivative, abenzophenone derivative, and an azo compound.

Further, examples of the thermal decomposition type polymerizationinitiator include a persulfate such as sodium persulfate, potassiumpersulfate, or ammonium persulfate; a peroxide such as hydrogenperoxide, t-butyl peroxide, or methyl ethyl ketone peroxide; and an azocompound such as 2,2′-azobis(2-amidinopropane) dihydrochloride, or2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride.

Furthermore, examples of the redox type polymerization initiator includea system using a reducing compound such as L-ascorbic acid or sodiumbisulfite together with the above persulfate or peroxide. It is alsopreferable to use the above photodecomposition type polymerizationinitiator and thermal decomposition type polymerization initiatortogether. Among these polymerization initiators, an azo polymerizationinitiator to generate N₂ by pyrolysis may be used to accelerate foaming.Further, an active energy ray such as an ultraviolet ray, an electronray, or a γ ray may be used singly or in combination with the abovepolymerization initiator.

The amount of the polymerization initiator used is preferably 0.0001 mol% to 1 mol % and more preferably 0.0005 mol % to 0.5 mol % with respectto 100 mol % of the monomer. When the amount to use thereof is 1 mol %or less, deterioration of color tone of the water-absorbing resin issuppressed, which is preferable. When the amount to use thereof is 0.5mol % or less, deterioration of color tone is further suppressed, whichis more preferable. Further, when the amount to use thereof is 0.0001mol % or more, an increase of a residual monomer is suppressed, which ispreferable. When the amount to use thereof is 0.0005 mol % or more, anincrease of a residual monomer is further suppressed, which is morepreferable.

(Adhesion Controlling Agent)

In order to sophisticatedly solve the problem of the present invention,an adhesion controlling agent specifically described in (2-3-2) isadded. The adhesion controlling agent may be added before, during, orafter the polymerization step, and specifically, is added in the step(iii) or before the step (iii).

(More Preferable Polymerization Method)

In the present invention, as a method for polymerizing a (meth)acrylicacid (salt)-based aqueous monomer solution, from the viewpoint of thephysical properties of the water-absorbing resin (for example, waterabsorption speed or liquid permeability), easiness of controllingpolymerization or the like, aqueous solution polymerization is employed.In particular, kneader polymerization or belt polymerization ispreferably employed, and continuous aqueous solution polymerization ismore preferably employed.

Preferable examples of the aqueous solution polymerization includecontinuous belt polymerization (disclosed in U.S. Pat. Nos. 4,893,999,6,241,928, US 2005/215734 A, and the like), continuous kneaderpolymerization and batch kneader polymerization (disclosed in U.S. Pat.Nos. 6,987,151, 6,710,141, and the like). Among these aqueous solutionpolymerizations, in the method described in JP 2002-212204 A, the timefor the polymerization step can be shortened by setting thepolymerization initiation temperature to a high temperature of 50° C. orhigher. Furthermore, the maximum reaching temperature can be suppressedby utilizing evaporative latent heat of water, and as a result, it ispossible to obtain a water-absorbing resin in which the main chainmolecular weight is high and the molecular weight distribution isnarrow, which is particularly preferable.

As an example of the preferred embodiment of the aqueous solutionpolymerization, the concentration of a monomer in the (meth)acrylic acid(salt)-based aqueous monomer solution is preferably 10 wt % to 80 wt %,more preferably 20 wt % to 60 wt %, and still more preferably 30 wt % to50 wt %. Incidentally, in Examples of the present application, themonomer concentration was 38 wt % and 39 wt %.

When the monomer concentration is in the above ranges, a load of themachine at the next gel-crushing step does not become excessively high,and the weight average particle diameter of the particulate hydrogel canbe efficiently controlled in a desired range.

Furthermore, in order to control GCA and FGBP in a desired range, it ispreferable to control the maximum reaching temperature duringpolymerization. Specifically, the maximum reaching temperature duringpolymerization is preferably 70° C. to 130° C., more preferably 80° C.to 120° C., and still more preferably 85° C. to 110° C.

Further, in the polymerization, if necessary, a chain transfer agentsuch as hypophosphorous acid (salt), for example, a chelating agent suchas trisodium diethylenetriamine pentaacetate or pentasodiumdiethylenetriamine pentaacetate, or the like may be further added to areaction system before or during polymerization in an amount ofpreferably 0 wt % to 3 wt % and more preferably 0.001 wt % to 1 wt %with respect to 100 wt % of the monomer.

(2-3) Gel-Crushing Step (Step (iii))

This is a step for crushing the crosslinked hydrogel polymer(hereinafter, referred to as “hydrogel” in some cases) obtained throughthe polymerization step and the like to obtain hydrogel in the state ofa particulate (hereinafter, referred to as “hydrogel particles” or“particulate hydrogel” in some cases). By grain refining of theparticulate hydrogel by gel-crushing of the hydrogel, particularly bygel-crushing using kneading up to a specific particle diameter range,GCA and vortex are improved.

Here, from the viewpoint of easiness of gel-crushing and drying of theparticulate hydrogel after gel-crushing, solids content of thecrosslinked hydrogel polymer is preferably 10 wt % to 80 wt %, morepreferably 20 wt % to 70 wt %, still more preferably 20 wt % to 60 wt %,still more preferably 30 wt % to 50 wt %, and particularly preferably 35wt % to 48 wt %.

When the solids content is less than 10 wt %, the moisture content ofthe particulate hydrogel after crushing becomes high, and thus energynecessary for drying and drying time are increased too. Therefore, thereis a concern in a cost increase and a decrease in production efficiency.In the meantime, when the solids content is more than 80 wt %, the gelis hardened so that a load to the pulverization apparatus is increased.So, there is a concern that the pulverization apparatus may be broken.

Incidentally, a method for adjusting the solids content of thecrosslinked hydrogel polymer to 10 wt % to 80 wt % is not particularlylimited, but for example, it may be performed by controlling the totalamount of monomers during polymerization.

The “weight average particle diameter of the particulate hydrogel(hydrogel particles)” after this gel-crushing step is 100 μm to 900 μm,preferably 120 μm to 870 μm, more preferably 130 μm to 860 μm, and stillmore preferably 140 μm to 850 μm, and may be 150 μm to 800 μm, 160 μm to700 μm, 170 μm to 600 μm, or 100 μm to 350 μm.

The “weight average particle diameter of the hydrogel particlesconverted to the dried product” after this gel-crushing step is 50 μm to650 μm, preferably 80 μm to 640 μm, more preferably 100 μm to 630 μm,still more preferably 110 μm to 600 μm, still more preferably 120 μm to500 μm, and particularly preferably 130 μm to 460 μm, and may be 140 μmto 400 μm.

Herein, the “weight average particle diameter (RB) of the hydrogelparticles converted to the dried product (solids content: 100 wt %)” canbe derived by the formula: “the weight average particle diameter (RA) ofthe hydrogel particles”×“(the solids content of the hydrogelparticles)^(1/3).”

It is described how to derive such a formula. The volume of the hydrogelparticles changes as a similar figure depending on the water absorptionamount (fluid retention capacity)thereof and the particle diameter alsochanges as a similar figure in accordance with the volume change.Herein, the description will be given assuming the hydrogel particlesand dried products thereof are spherical.

A volume V of the sphere can be expressed as (π×R³)/6 when the diameterof the sphere is designated as R. Similarly, a volume VA can beexpressed as {π×(RA)³)/6 when the hydrogel particles are assumed as asphere. Meanwhile, a volume VB of the dried product can be expressed as{π×(RB)³)/6 when the solids content is regarded as 100 wt %. Therefore,the ratio VA:VB can be established as {π×(RA)³)/6:{π×(RB)³)/6, andRB=RA×(VB/VA)^(1/3) can be established.

Herein, the volumes VA and VB of the hydrogel particles are proportionalto the reciprocal of the solids content. For example, when the solidscontent of the hydrogel particles is 40 wt %, assuming the solidscontent of the dried product as 100 wt %, the volume of the hydrogelparticles is 2.5 times (=100/40).

When the solids content of the hydrogel particles is designated as SAand the solids content of hydrogel particles is designated as SB, theratio VA:VB can be expressed as (1/SA):(1/SB). Therefore, when each of1/SA and 1/SB is used for calculation instead of VA and VB in theaforementioned RB=RA×(VB/VA)^(1/3), the following equation can beestablished: RB=RA×(SA/SB)^(1/3). Herein, when SB is 100, the followingequation can be derived: RB=RA×(SA/100)^(1/3).

For example, specifically, when 40 wt % (solids content) of the gel iscrushed, 100 μm to 900 μm of the particulate hydrogel is calculated as74 μm to 663 μm converted to the dried product.

(Gel-Crusher)

In the present invention, it is important that the weight averageparticle diameter of the particulate hydrogel (hydrogel particles) iscontrolled to the above-mentioned range, and a means for achieving themis not particularly limited. Examples thereof include a batch type orcontinuous type double-arm kneader, a gel-crusher provided with aplurality of rotary stirring blades, a uniaxial extruder, a biaxialextruder, a meat chopperor the like. In particular, a screw extruder(for example, a meat chopper) having a porous plate at an end thereof ispreferable, which includes a screw extruder disclosed in JP 2000-063527A, for example.

In particular, the gel-crusher used in this step is more preferably ascrew extruder, and still more preferably a screw extruder (for example,a meat chopper) having a porous plate at one end of a casing. Forexample, screw extruders disclosed in JP 2000-63527 A, WO 2015/030129 A,and WO 2015/030130 A are exemplified. Hereinafter, an example of a screwextruder used in this step is described. The same apparatus is also usedin these Examples.

A screw extruder for use in the present step is constituted by, forexample, a casing, a table, a screw, a feed orifice, a hopper, adischarge orifice, a porous plate, a rotary blade, a ring, a backflowpreventer, a motor, a ridge, and/or the like. The casing is in the formof a cylinder and has the screw therein. The casing has the dischargeorifice at one end thereof, through which the crosslinked hydrogelpolymer is extruded and subjected to gel-crushing, and has the porousplate placed short of the discharge orifice. The casing has, at theother end, the motor and a drive system and the like for rotation of thescrew. There is the table under the casing and thereby the screwextruder can sit stably. The casing has, on the other hand, the feedorifice at the top thereof, through which the crosslinked hydrogelpolymer is fed. The feed orifice is provided with the hopper for easyfeeding of the crosslinked hydrogel polymer. The shape and size of thecasing are not particularly limited, provided that the casing has aninner surface in the form of a cylinder that corresponds to the shape ofthe screw. The speed of the screw varies depending on the shape of thescrew extruder and is not particularly limited, but is preferablyadjustable as is described later. Furthermore, for example, the screwextruder may include the backflow preventer near the discharge orificeand may have the ridge on the screw. The arrangements of thesecomponents, materials for these components, sizes of these components,materials for the backflow preventer and the rotary blades attached tothe screw, and all other arrangements related to the screw extruder maybe determined in accordance with the method disclosed in theabove-mentioned Japanese Patent Application Publication, Tokukai, No.2000-63527, WO 2015/030129 A, and WO 2015/030130.

For example, the backflow preventer is not limited to a particular kind,provided that the backflow of the crosslinked hydrogel polymer at ornear the discharge orifice is prevented by the structure of the backflowpreventer. The backflow preventer is, for example, a ridge in the formof a spiral or ridges in the form of concentric circles on the innerwall of the casing, a linear, particulate, spherical, or angularprojection parallel to the screw, or the like. When the pressure at ornear the discharge orifice increases as the gel-crushing proceeds, thecrosslinked hydrogel polymer tries to flow back toward the feed orifice.Providing the backflow preventer makes it possible to prevent thebackflow of the crosslinked hydrogel polymer while performinggel-crushing of the crosslinked hydrogel polymer.

In the present invention, the particle diameter after gel-crushing issignificantly small. A method for adjusting the particle diameter inthis way is not particularly limited, but in the case of using a screwextruder in which a porous plate is provided at one edge of the casing,it is necessary to change the size of the device according to the amountof the crosslinked hydrogel polymer to be subjected to gel-crushingtreatment. At this time, it is possible to adjust the size of the deviceby appropriately adjusting a thickness of the porous plate, a porediameter of the porous plate, an open area of the porous plate, therotation speed of a screw axis, a feed rate of the crosslinked hydrogelpolymer, and the like in accordance with an apparatus to be used.

In regard to the porous plate at the exit of the casing of thegel-crusher, the thickness, pore diameter, and the open area of theporous plate may be selected as appropriate depending on the volume perunit time of the crosslinked hydrogel polymer crushed by thegel-crusher, the properties of the crosslinked hydrogel polymer, and thelike and are not particularly limited. The thickness of the porous plateis preferably in a range of 3.5 mm to 100 mm and more preferably in arange of 6 mm to 80 mm. The diameter is preferably 30 mm to 1,500 mm andmore preferably 40 mm to 1,000 mm.

Further, the pore diameter of the porous plate is preferably in a rangeof 1.0 mm to 50 mm and more preferably in a range of 2.0 mm to 30 mm.

Furthermore, the open area of the porous plate is preferably in a rangeof 10% to 80%, more preferably in a range of 20% to 60%, and still morepreferably in a range of 25% to 55%.

Further, the number of pores is preferably 10 to 2,000 and morepreferably 20 to 1,000.

It is noted that, in a case where a plurality of porous plates havingdifferent pore diameters (mm) are used, the simple average of the porediameters of the porous plates is used as the pore diameter of theporous plates of the gel-crusher. The shape of the pore is preferably acircle. In a case where the shape of the pore is a shape other than acircle, such as a square, an oval, a slit, or the like, the area of thepore is converted into the area of a circle and this diameter of thecircle is used as a pore diameter (mm).

Further, the outer diameter of the screw axis is preferably 10 mm to2,000 mm and more preferably 20 mm to 1,000 mm.

Further, the feed rate of the crosslinked hydrogel polymer is preferably0.10 kg/min to 550 kg/min and more preferably 0.12 kg/min to 500 kg/min.

If the thickness of the porous plate is less than 3.5 mm, the porediameter of the porous plate is more than 50 mm, and/or the open area ofthe porous plate is more than 80%, there is a case where the crosslinkedhydrogel polymer cannot be gel-crushed to particulate hydrogel having adesired particle diameter. On the contrary, if the thickness of theporous plate is more than 100 mm, the pore diameter of the porous plateis less than 1.0 mm, and/or the open area of the porous plate is lessthan 10%, such a porous plate may apply excessive shearing andcompressive forces to the crosslinked hydrogel polymer, resulting in areduction in physical properties of the crosslinked hydrogel polymer.

When the feed rate of the crosslinked hydrogel polymer is 0.10 kg/min orless, excessive shearing stress and compressive force are applied to thecrosslinked hydrogel polymer so that physical properties may bedeteriorated. On the other hand, when the feed rate of the crosslinkedhydrogel polymer is more than 550 kg, there is a case where thecrosslinked hydrogel polymer cannot be gel-crushed to particulatehydrogel having a desired particle diameter.

Further, as described above, according to the present invention, theparticle diameter after gel-crushing is significantly small. A methodfor adjusting the particle diameter in this way is not particularlylimited, but the method may be performed by setting gel grinding energyto an appropriate value.

The gel grinding energy (GGE (1)) is preferably 10 J/g to 500 J/g, morepreferably 15 J/g to 400 J/g, still more preferably 20 J/g to 300 J/g,still more preferably 45 J/g to 250 J/g, and particularly preferably 25J/g to 200 J/g.

In a case where the gel grinding energy (GGE (1)) is less than 10 J/g,there is a case where the crosslinked hydrogel polymer cannot begel-crushed to particulate hydrogel having a desired particle diameter.On the other hand, in a case where the gel grinding energy (GGE (1)) ismore than 500 J/g, a load to a pulverization apparatus becomes large sothat there is a concern that the pulverization apparatus may be brokenin a continuous operation. Moreover, excessive shearing stress andcompressive force are applied to the crosslinked hydrogel polymer sothat the amount of the water soluble component produced may be increasedor deterioration of physical properties such as a decrease in CRC andAAP may occur.

The gel grinding energy (GGE (2)) is preferably 5 J/g to 300 J/g, morepreferably 6 J/g to 280 J/g, still more preferably 8 J/g to 260 J/g,still more preferably 9 J/g to 250 J/g, and still more preferably 10 J/gto 240 J/g.

In a case where the gel grinding energy (GGE (2)) is less than 5 J/g,there is a case where the crosslinked hydrogel polymer cannot begel-crushed to particulate hydrogel having a desired particle diameter.On the other hand, in a case where the gel grinding energy (GGE (2)) ismore than 300 J/g, excessive shearing stress and compressive force areapplied to the crosslinked hydrogel polymer so that the amount of thewater soluble component produced may be increased or deterioration ofphysical properties such as a decrease in CRC and AAP may occur.

Incidentally, in a case where pulverization is performed multiple times,GGE is obtained by adding the gel grinding energy at the time of eachpulverization.

GGE Calculation Method

“Gel Grinding Energy” (GGE (1), GGE (2))

In an embodiment of the present invention, the term “gel grindingenergy” denotes mechanical energy per unit weight (unit weight of acrosslinked hydrogel polymer) required for a gel-crusher to crush thecrosslinked hydrogel polymer. It is noted that the “gel grinding energy”does not include energy to heat or cool a jacket or energy of introducedwater or steam. It is noted that the “gel grinding energy” is referredto as “GGE(1)” for short. The GGE is calculated using the followingEquation (1) in a case where a gel-crusher is driven by three-phaseelectric power.

[Mathematical Formula 1]

GGE (1) [J/g]={√3×voltage×current value during gel-crushing×powerfactor×motor efficiency}/{weight of crosslinked hydrogel polymer fedinto gel-crusher per second}  (Equation 1)

In the equation, the “power factor” and the “motor efficiency” arevalues inherent to the gel-crusher and vary depending on operationconditions and the like of the gel-crusher, and may have a value of 0to 1. It is possible to know the characteristic values by inquiring themfrom a manufacturer of the device or the like. In a case where thegel-crusher is driven by single-phase electric power, the GGE can becalculated using a modified equation where “√{square root over (3)}” inthe above equation is changed to “1”. In the equation, the unit ofvoltage is [V], the unit of current is [A], and the unit of weight ofthe crosslinked hydrogel polymer is [g/s].

The mechanical energy applied to a crosslinked hydrogel polymer isimportant in an embodiment of the present invention. Therefore, it ispreferable that the gel grinding energy be calculated excluding thevalue of current flowing while the gel-crusher is in the idle state.Especially in a case where a plurality of gel-crushers are used to crushgel, the total value of current during the idle state is large and thusa calculation excluding the values of current during the idle state ispreferred. The gel grinding energy in this case is calculated using thefollowing Equation (2). It is noted that, for distinction from theabove-described GGE(1), the gel grinding energy calculated using thefollowing Equation (2) is represented as GGE (2).

[Mathematical Formula 2]

GGE (2) [J/g]={√{square root over (3)}×voltage×(current value duringgel-crushing−current value during idle state)×power factor×motorefficiency}/{weight of crosslinked hydrogel polymer fed into gel-crusherper second}  (Equation 2)

According to the preferred embodiment of the present invention, thecrosslinked hydrogel polymer is gel-crushed to obtain hydrogelparticles, the hydrogel particles are dried to obtain a dried product,and then the dried product is “pulverized.” In this way, since“pulverization” is performed at the subsequent step, it is consideredthat it is not necessary to take the trouble to pulverize the“crosslinked hydrogel polymer,” which is difficult to pulverize,multiple times or to decrease the particle diameter after gel-crushingto be significantly small by setting the gel grinding energy to morethan a predetermined degree.

In this regard, the present invention can provide a poly(meth)acrylicacid (salt)-based particulate water-absorbing agent enabling reductionin re-wet amount and having an excellent speed of incorporating liquidand a production method therefor by employing a unique configuration inwhich the particle diameter after gel-crushing is adjusted to besignificantly small and an adhesion controlling agent is used.

(Gel-Cutting or Gel-Grinding before Gel-Crushing)

In a case where the polymerization step is performed by beltpolymerization, a crosslinked hydrogel polymer may be chopped or brokenduring or after polymerization, preferably after polymerization, to asize of about several tens of centimeters prior to the gel-crushing.Herein, gel-cutting or gel-grinding is a primary treatment performingcutting or grinding in such a size that the cut or ground product can becontinuously supplied to a crusher (for example, a primary treatment of1,000 cm³ or less or 1,000 cm² or less in plane). On the other hand,gel-crushing is different from gel-cutting or gel-grinding in that finegranulating is performed (particularly, fine granulating to have aweight average particle diameter of 50 micron to 650 micron converted tothe dried product).

This operation makes it easily to feed the crosslinked hydrogel polymerinto the gel-crusher and thus possible to more smoothly perform thegel-crushing step. It is noted that the chopping or breaking ispreferably performed by a method that enables chopping or breaking ofthe crosslinked hydrogel polymer without kneading the crosslinkedhydrogel polymer, and is, for example, chopping or breaking or the likeusing a guillotine cutter. The size and shape of the chopped or brokencrosslinked hydrogel polymer are not particularly limited, provided thatthe crosslinked hydrogel polymer can be fed into the gel-crusher, but asfor the shape, a block shape is preferable.

(Polymerization Rate of Crosslinked Hydrogel Polymer beforeGel-Crushing)

The gel-crushing in the present invention is performed duringpolymerization and/or after polymerization and is preferably performedon the crosslinked hydrogel polymer after polymerization. Incidentally,as polymerization in which gel-crushing is performed duringpolymerization, kneader polymerization is exemplified, but gel-crushingmay be performed further after polymerization. Further, aspolymerization in which gel-crushing is performed after polymerization,belt polymerization or stand-still aqueous solution polymerization(aqueous solution polymerization under substantially no stirring) in atank is preferably exemplified, but polymerization is not particularlylimited to these polymerization examples.

The polymerization rate of the crosslinked hydrogel polymer to besubjected to gel-crushing is preferably 90 mol % or more, morepreferably 93 mol % or more, even more preferably 95 mol % or more,particularly preferably 97 mol % or more. The upper limit is preferably99.5 mol %. The crosslinked hydrogel polymer to be subjected togel-crushing having a polymerization rate of 90 mol % or more ispreferred, because residual monomers contained in the resultingwater-absorbing resin become small in number. Incidentally, also in acase where gel-crushing is performed during polymerization, it is onlynecessary to continue polymerization as described below until theabove-described polymerization rate is attained. As used herein, thepolymerization rate, which is also referred to as conversion rate, meansa value calculated from polymer content of the crosslinked hydrogelpolymer and unreacted monomer content.

The polymerization rate of the crosslinked hydrogel polymer to besubjected to gel-crushing preferably falls within the above range.However, in a case where gel-crushing is performed duringpolymerization, such as performing kneader polymerization, it is assumedthat the gel-crushing step starts when the aqueous monomer solution hasturned into a “sufficiently gelled state”.

For example, in a case where the kneader polymerization is employed, theaqueous monomer solution changes into a crosslinked hydrogel polymer aspolymerization progresses. Specifically, an aqueous monomer solution isstirred at the initiation of polymerization, a crosslinked hydrogelpolymer having a certain viscosity and a low degree of polymerization isstirred during polymerization, gel-crushing of part of the crosslinkedhydrogel polymer starts as the polymerization progresses, andgel-crushing is performed in the last half or in the final stage of thepolymerization, sequentially in a single region. Therefore, for cleardistinction between “stirring of an aqueous monomer solution” at theinitiation of the polymerization and “gel-crushing” in the final stageof the polymerization, it is judged that a transition to thegel-crushing step has occurred when the “sufficiently gelled state” isreached.

The term “sufficiently gelled state” denotes a state in which thecrosslinked hydrogel polymer can be grain-refined by applying shearingforce and which occurs when or after the maximum polymerizationtemperature (polymerization peak temperature) is reached. The term“sufficiently gelled state” also denotes a state in which thecrosslinked hydrogel polymer can be grain-refined by applying shearingforce and which occurs when or after the polymerization rate of monomersin the aqueous monomer solution reaches preferably 90 mol % or more,more preferably 93 mol % or more, even more preferably 95 mol % or more,particularly preferably 97 mol % or more. That is, in the gel-crushingstep of an embodiment of the present invention, a crosslinked hydrogelpolymer having a monomer polymerization rate falling within the aboverange is preferably subjected to the gel-crushing. It is noted that, ina case of a polymerization reaction that shows no polymerization peaktemperature (for example, in a case where entire polymerization proceedsat constant temperature, a case where the polymerization temperaturecontinues to rise, or the like), “sufficiently gelled state” isdetermined on the basis of the fact that the polymerization rate of themonomers becomes preferably 90 mol % or more, more preferably 93 mol %or more, still more preferably 95 mol % or more, and particularlypreferably 97 mol % or more.

(Operating Conditions of Gel-Crusher)

In a case where the gel-crusher for use in the gel-crushing step of anembodiment of the present invention is a screw extruder (for example, ameat chopper), the rotation speed of a screw shaft of the screw extrudercannot be specified by a particular value because the peripheral speedof the impeller blades varies depending on the inner diameter of thecasing of the screw extruder, the outer diameter of the screw axis, andthe like. The shaft rotation speed is preferably 80 rpm to 500 rpm, morepreferably 90 rpm to 400 pm, even more preferably 100 rpm to 300 rpm.

Incidentally, the inner diameter of the casing is suitably about 15 mmto 2,500 mm and more suitably about 25 mm to 1,500 mm.

In a case where the shaft rotation speed is 80 rpm or more, shearing andcompressing forces necessary for gel-crushing are achieved. In a casewhere the shaft rotation speed is 500 rpm or less, the shearing andcompressive forces applied to the crosslinked hydrogel polymer are notexcessive and therefore physical properties are not likely todeteriorate, and the gel-crusher does not experience large load and thusis not prone to breakage of the device.

Meanwhile, in a case where the rotation speed of the axis is less than80 rpm, shearing stress and compressive force required for thegel-crushing are difficult to obtain. In a case where the rotation speedof the axis is more than 500 rpm, shearing stress and compressive forceto be applied to the crosslinked hydrogel polymer become excessive. Thisdeteriorates the physical properties and increases load on thegel-crusher thereby damaging the device.

Further, the rate of a periphery of a rotational blade at this time ispreferably 0.5 m/s to 10 m/s and more preferably 0.5 m/s to 8 m/s.

Further, the gel-crusher in the present invention is heated or kept tohave a temperature preferably in a range of 30° C. to 120° C. and morepreferably 40° C. to 100° C. so as to prevent the crosslinked hydrogelpolymer from adhering thereto.

Further, the temperature of the gel-crusher in the present invention isalso suitably set to be in a temperature range of the gel temperaturedescribed below.

(Number of Times of Gel-Crushing Treatment)

In the present invention, the number of times of the gel-crushingtreatment is not particularly limited as long as the particle diameterafter gel-crushing is significantly small, but according to anembodiment of the present invention, the number of times of thegel-crushing treatment is multiple times.

In a case where the gel-crushing is performed multiple times, a methodin which treatment is performed multiple times with use of onegel-crusher may be employed or treatment may be continuously performedby disposing a plurality of gel-crushers in series. By performing thegel-crushing multiple times, the weight average particle diameter of theparticulate hydrogel can be adjusted in a desired range by gel-crushingunder relatively mild conditions, the physical properties of thewater-absorbing resin are less likely to degrade, and a load applied toa gel-crusher is small so that there is no concern that the device isdamaged.

Gel-crushers in the case of performing the treatment multiple times donot need to be the same type of machine, and different types of machinemay be combined, or even in the same type of machine, setting conditionand operation conditions may be changed.

Therefore, according to the preferred embodiment of the presentinvention, the gel-crushing step is performed with use of a plurality ofgel-crushers. According to such an embodiment, the technical effect oflowering a load applied to one crusher is achieved.

The preferable number of times of treatment in the case of performingthe treatment multiple times is preferably 2 to 5 times, more preferably2 to 4 times, and still more preferably 2 to 3 times. Incidentally,performing the gel-crushing twice indicates that hydrogel particlesdischarged from a discharge port of a gel-crusher are introduced againinto the gel-crusher and then the gel-crushing is performed. The sameapplies to the case of performing the gel-crushing three or more times.

Further, if the particle diameter after gel-crushing is significantlysmall, it is sufficient to perform the gel-crushing once. In the case ofperforming the gel-crushing once, costs of introduction and maintenanceof a gel-crusher are suppressed and space for production facility isalso suppressed. Further, energy burden can be reduced, which ispreferable.

(Gel Temperature)

Gel temperature, specifically, the temperature of a crosslinked hydrogelpolymer that has not been subjected to gel-crushing, is preferably 40°C. to 120° C., more preferably 50° C. to 120° C., even more preferably52° C. to 110° C., even more preferably 48° C. to 80° C., particularlypreferably 56° C. to 70° C., from the viewpoint of particle size controland physical properties. Incidentally, the gel temperature may be 65° C.to 110° C.

Further, the numerical value of the gel temperature may be applied tothe temperature of the gel-crusher.

In a case where the gel temperature is lower than 40° C., in terms ofthe characteristics of the crosslinked hydrogel polymer, adherabilitybecomes relatively high. Thus, it is difficult to control the particleshape and the particle size distribution during gel-crushing. Further,in a case where gel temperature is higher than 120° C., evaporation ofwater from the gel becomes significant so that the solids content of thehydrogel is changed. This makes the crushing difficult. Thus, it isdifficult to control the particle diameter and the particle shape of theparticulate hydrogel. Such a gel temperature can be appropriatelycontrolled by the polymerization temperature, heating, warming, orcooling after polymerization, and the like.

(Gel CRC)

The gel CRC of a crosslinked hydrogel polymer that has not beensubjected to gel-crushing and the particulate hydrogel (hydrogelparticles) after gel-crushing are preferably one of the values is 25 g/gto 50 g/g, and more preferably, both values are 25 g/g to 50 g/g, morepreferably 26 g/g to 45 g/g, and still more preferably 27 g/g to 40 g/g.In a case where the gel CRC is in the above ranges, particle shape andparticle size distribution are easy to control when gel-crushing isperformed, and thus such a case is preferred. Such a gel CRC can beappropriately controlled by the amount of a crosslinking agent addedduring polymerization and other parameters such as polymerizationconcentration. It is noted that it is well known that a water absorbentresin having a high CRC is preferred. However, in a case where the gelCRC exceeds the above range, in some cases, it may be difficult tocontrol the particle shape and the particle size distribution.

In a case where there is a change between CRC of the crosslinkedhydrogel polymer before gel-crushing and CRC of the hydrogel particlesafter gel-crushing, the change in CRC in the gel-crushing step in whichthe crosslinked hydrogel polymer is crushed to obtain hydrogel particles(a value obtained by subtracting CRC of the hydrogel particles aftergel-crushing from CRC of the crosslinked hydrogel polymer beforegel-crushing, unit is g/g) is preferably −10 to +10, more preferably −8to +8, still more preferably −6 to +6, particularly preferably −5 to +5,and most preferably −4 to +4.

When the change in CRC becomes larger than −10 and CRC of the resultinghydrogel particles becomes small, it is difficult to adjust CRC in thedrying step. When the change in CRC becomes larger than +10, damage tothe gel in the gel-crushing step is increased to increase the elutedcomponent. Thus, there is a concern that GCA and FGBP of thewater-absorbing agent are lowered.

The gel CRC of the crosslinked hydrogel polymer is determined by themeasurement method which is described later in section of [Examples],after cutting and grain-refining the crosslinked hydrogel polymer thathas not been subjected to gel-crushing into pieces 1 mm to 3 mm on aside with the use of scissors, a cutter, or the like. And, the gel CRCof the particulate hydrogel (hydrogel particles) after gel-crushing isdetermined by the measurement method which is described later in sectionof [Examples]without being cut or fragmented by the measurement methodthat is described in [Examples] described later.

(Solids Content of Resin before and after Gel-Crushing)

In the present invention, from the viewpoint of physical properties, thesolids content of the resin of the crosslinked hydrogel polymer beforegel-crushing is preferably 10 wt % to 80 wt %, more preferably 20 wt %to 60 wt %, still more preferably 30 wt % to 55 wt %, still morepreferably 33 wt % to 50 wt %, and particularly preferably 36 wt % to 46wt %.

In the present invention, from the viewpoint of physical properties, thesolids content of the resin of the particulate hydrogel (hydrogelparticles) after gel-crushing is 10 wt % to 80 wt %, preferably 20 wt %to 60 wt %, more preferably 30 wt % to 55 wt %, still more preferably 33wt % to 50 wt %, and still more preferably 36 wt % to 46 wt %.

By adjusting the solids content of the resin of the particulate hydrogel(hydrogel particles) after gel-crushing in the above range, preferably,by adjusting the solids content of the resin of the crosslinked hydrogelpolymer before gel-crushing in the above range, damage (an increase inwater soluble component, or the like) caused by drying is decreased.

Incidentally, the solids content of the resin of the hydrogel particlesafter gel-crushing can be appropriately controlled, if necessary, byadding water before gel-crushing or during gel-crushing, by evaporatingmoisture by heating during gel-crushing, or the like.

In a case where there is a change in solids content between thecrosslinked hydrogel polymer before gel-crushing and the hydrogelparticles after gel-crushing, the change in solids content in thegel-crushing step in which the crosslinked hydrogel polymer is crushedto obtain hydrogel particles (a value obtained by subtracting the solidscontent of the hydrogel particles after gel-crushing from the solidscontent of the crosslinked hydrogel polymer before gel-crushing, unit iswt %) is preferably −10 to +10, more preferably −8 to +8, still morepreferably −6 to +6, particularly preferably −5 to +5, and mostpreferably −4 to +4. Herein, the minus symbol means that the solidscontent is decreased (the moisture content is increased), and plus (+)symbol means that the solids content is increased (the moisture contentis decreased).

When the change in solids content becomes larger than −10, a load to thedrying step is increased by an increase in the moisture content of thehydrogel particles. This makes sufficient drying difficult or a largerquantity of thermal energy necessary. That is, the production efficiencyis degraded.

When the change in solids content becomes larger than +10, damage to thegel in the gel-crushing step is increased to increase the elutedcomponent. Thus, there is a concern that GCA and FGBP of thewater-absorbing agent are lowered.

(Use of Water)

In the gel-crushing step of an embodiment of the present invention,water may be added to a crosslinked hydrogel polymer before subjectingthe crosslinked hydrogel polymer to gel-crushing. It is assumed in anembodiment of the present invention that “water” includes at least oneof the solid, liquid, and gaseous forms.

How and when water is added are not particularly limited, provided thatwater is fed to the gel-crusher while a crosslinked hydrogel polymerresides in the gel-crusher. Alternatively, a crosslinked hydrogelpolymer to which water has been added may be fed into the gel-crusher.Furthermore, the water is not limited to “water alone” and may be in theform of a mixture of water and another additive (for example,surfactant, base for neutralization) or a solvent other than water.

However, in this case, the water content is preferably 90 weight % to100 weight %, more preferably 99 weight % to 100 weight %, even morepreferably substantially 100 weight %.

In an embodiment of the present invention, the water in at least one ofthe solid, liquid, and gaseous forms may be used, but the water inliquid and/or gaseous form is preferred from the viewpoint ofhandleability. The amount of water fed is preferably 0 parts by weightto 4 parts by weight or less, more preferably 0 parts by weight to 2parts by weight or less, relative to 100 parts by weight of thecrosslinked hydrogel polymer. In a case where the amount of the waterfed is more than 4 parts by weight, this may cause some problems such asundried materials left undried even after drying.

In a case where the water is fed in liquid form, the temperature of thewater when fed is preferably 10° C. to 100° C., more preferably 40° C.to 100° C. The water in the form of liquid is appropriately added bymeans of spray, mist, showering, droplet, a straight pipe, or the like.In a case where the water is fed in gaseous form, the temperature of thewater fed is preferably 100° C. to 220°, more preferably 100° C. to 160°C., even more preferably 100° C. to 130° C. It is noted that, when wateris fed in gaseous form, a method of preparing the water in gaseous formis not particularly limited. The water in gaseous form may be preparedby, for example: a method using water vapor generated from heat made bya boiler; a method using water in gaseous form released from the surfaceof water ultrasonically vibrated; or the like. In an embodiment of thepresent invention, in a case where water is fed in gaseous form, thewater is preferably water vapor with higher pressure than atmosphericpressure, more preferably water vapor generated by a boiler.

In order to solve the problem of the present invention, it is preferableto employ aqueous solution polymerization rather than reverse phasesuspension polymerization in which gel-crushing is not necessary, and itis particularly preferable to employ aqueous solution polymerization inwhich gel-crushing is performed during polymerization (for example,kneader polymerization) or after polymerization (for example, beltpolymerization and further, if necessary, kneader polymerization).

(2-3-2) Addition of Adhesion Controlling Agent

In order to more sophisticatedly solve the problem of the presentinvention, the gel contains an adhesion controlling agent duringgel-crushing. In other words, it is sufficient to add an adhesioncontrolling agent before the gel-crushing is completely finished. Fordoing this, an adhesion controlling agent is added in at least one stepof the step (i): the step for preparing a (meth)acrylic acid(salt)-based aqueous monomer solution, the step (ii): the polymerizationstep, and the step (iii): the gel-crushing step, or an adding step maybe provided between the step (i) and the step (ii) or between the step(ii) and the step (iii). As a step performed between the step (i) andthe step (ii), for example, a step for storing and transporting theprepared (meth)acrylic acid (salt)-based aqueous monomer solution isexemplified, and as a step performed between the step (ii) and the step(iii), for example, a step for aging a hydrogel-like polymer isexemplified. In this way, by containing an adhesion controlling agent inthe inside and/or the surface of the hydrogel particles in the step(iii) or before the step (iii), the desired effect of the presentinvention can be exerted.

Further, the step (iii) is a step for gel-crushing the crosslinkedhydrogel polymer during polymerization or after polymerization to obtainhydrogel particles. However, adding an adhesion controlling agent to“the crosslinked hydrogel polymer” before performing gel-crushing or “aproduct obtained by cutting or grinding the crosslinked hydrogelpolymer” is also encompassed in the concept of “adding an adhesioncontrolling agent in the step (iii).”

The adhesion controlling agent may be in the form of liquid or solid,and may be added without any change or may be added in a state of asolution or a suspension.

Further, when the adhesion controlling agent is a radicallypolymerizable adhesion controlling agent having an unsaturated bond, inthe case of adding the radically polymerizable adhesion controllingagent in the step (i) or the step (ii), the radically polymerizableadhesion controlling agent may be consumed by reaction duringpolymerization so that the radically polymerizable adhesion controllingagent may not remain. Furthermore, in the case of addition in the step(iii), the radically polymerizable adhesion controlling agent may not besufficiently consumed to remain in a final product, which causescoloration. Thus, the adhesion controlling agent is preferablynon-radically polymerizable.

In the case of adding the adhesion controlling agent in the state of asolution or, in the case of adding the adhesion controlling agent in thestate of a suspension, a solvent and a dispersion medium are notparticularly limited, but water or alcohol is preferable and water isparticularly preferable.

The concentration of the adhesion controlling agent in the case ofaddition in the state of a solution or a suspension is preferably 0.1 wt% to 99 wt %, more preferably 0.1 wt % to 75 wt %, and still morepreferably 0.1 wt % to 50 wt %.

The temperature when the adhesion controlling agent is added in thestate of a solution is a melting point or higher and boiling point orlower, and further, is used at 0° C. to 100° C. and 20° C. to 50° C. Forimproving solubility, if necessary, it may be heated.

(Amount of Adhesion Controlling Agent Added)

The amount of the adhesion controlling agent added is not particularlylimited, and may be determined in consideration of the type of theadhesion controlling agent.

Although depending on the type of the adhesion controlling agent, theamount of the adhesion controlling agent added is preferably 0.01 wt %to 5 wt %, more preferably 0.02 wt % to 3 wt %, and still morepreferably 0.03 wt % to 2 wt % with respect to the raw material monomerof the crosslinked hydrogel polymer.

Incidentally, when the adhesion controlling agent is added in the step(iii), the amount of the adhesion controlling agent added is regarded asthe amount of the adhesion controlling agent added with respect to theraw material monomer and the raw material monomer is not the remainingraw material monomer but the raw material monomer used in the step (i)for preparing.

Therefore, according to the preferred embodiment of the presentinvention, the amount of the adhesion controlling agent added is 0.01 wt% to 5 wt % with respect to the raw material monomer of the crosslinkedhydrogel polymer. When the addition amount is less than these lowerlimits, the adhesion control effect is difficult to confirm. When theaddition amount is more than these upper limits, improvement in theadhesion prevention effect does not match with the addition amount,which is not economical.

The amounts of (a) the polyol and (b-1) the glycidyl-modified polyol,which is described below, added may be in the above range of the amountof the adhesion controlling agent added with respect to the raw materialmonomer of the crosslinked hydrogel polymer, but are preferably 0.01 wt% to 5 wt %, more preferably 0.02 wt % to 3 wt %, still more preferably0.03 wt % to 2 wt %, still more preferably 0.1 wt % to 1.8 wt %, andparticularly preferably 0.2 wt % to 1.5 wt %.

When the addition amount is less than these lower limits, the adhesioncontrol effect is difficult to confirm. When the addition amount is morethan these upper limits, improvement in the adhesion prevention effectdoes not match with the addition amount, which is not economical.

Further, the amounts of (b-2) the alkylene oxide adduct of higheralcohol, (b-3) the alkylene oxide adduct of the polyhydric alcohol fattyacid ester, (c) side-chain and/or terminal polyether-modifiedpolysiloxane, (d) the alkylene oxide adduct of higher aliphatic amine,(e) the alkylaminobetaine, (f) the alkylamine oxide, (g) the sulfuricacid ester salt of the higher alcohol alkylene oxide adduct, (h) thealkyl diphenyl ether sulfonate, and (i) the ammonium salt, which isdescribed below, added may be in the above range of the amount of theadhesion controlling agent added with respect to the crosslinkedhydrogel polymer, but is preferably 0.01 wt % to 5 wt %, more preferably0.01 wt % to 2 wt %, still more preferably 0.01 wt % to 1 wt %, andparticularly preferably 0.01 wt % to 0.5 wt %.

When the addition amount is less than these lower limits, the adhesioncontrol effect is difficult to confirm. When the addition amount is morethan these upper limits, improvement in the adhesion prevention effectdoes not match with the addition amount, which is not economical.

Incidentally, in this Example, the raw material monomer is acrylic acidand sodium acrylate.

The adhesion controlling agent described in the present invention ispresent on the surface of the crosslinked hydrogel polymer and/or thehydrogel particles during gel-crushing, therefore the adhesioncontrolling agent is an agent capable of lowering adhesion betweengel-crosslinked hydrogel polymers, between hydrogel particles, orbetween dried products ; adhesion between a crosslinked hydrogel polymerand hydrogel particles; or adhesion between hydrogel particles and adried product after gel-crushing or after drying.

As an index of adhesion lowering, as compared to a case where theadhesion controlling agent is not added and a case where thewater-absorbing resin powder after gel-crushing have the same particlesize (water-absorbing resin powder passing through a sieve having a meshsize of 500 μm and not passing through a sieve having a mesh size of 425μm), it appeared that the BET specific surface area is increased, or thevortex is shortened by 3 seconds or longer, more preferably 5 seconds orlonger, and still more preferably 7 seconds or longer. In this Example,comparison is carried out with “the same particle size.”

As described below in (2-4) the drying step, in the present invention,it is preferable that the weight average particle diameter of thewater-absorbing agent be smaller than the weight average particlediameter of the hydrogel particles after gel-crushing so that the shapein which primary particles are granulated more often appears in theresulting water-absorbing agent. That is, each particle of the primaryparticles is in loosely contact with each other (the contact area isrelatively small). Therefore, the surface area can be increased so thatdesired physical properties can be achieved.

On the other hand, in a case where the adhesion controlling agent is notadded, since adhesion cannot be lowered, granulation excessivelyproceeds so that particles are difficult to exist as primary particles,and thus desired physical properties are difficult to achieve.

The value obtained by dividing the weight average particle diameter ofthe water-absorbing agent by the weight average particle diameter of thehydrogel particles after gel-crushing converted to the dried product ispreferably 0.40 to 10.0, more preferably 0.45 to 5.0, and still morepreferably 0.50 to 4.0.

The hydrogel particles obtained by adding an adhesion controlling agentimproves slippage by controlling adhesion between the hydrogelparticles. For this reason, the hydrogel particles undergoing throughthe gel-crushing step is easy to flow in a drier, the treatment amountcan be increased, and productivity can be improved. Further, when thegel is easy to flow, thickness unevenness in the drier is lowered, dryunevenness during drying is lowered, and the physical properties of theresulting dried product are stable. Thus, it is more preferable to usean adhesion controlling agent.

When the strength of the gel-dried product obtained through the dryingstep (iv) is too high, in the pulverizing step of the dried productusing a crusher of the pulverizing and classification steps (step v), alarge load is applied to the crusher so that the lifespan of the devicemay be shortened. For this reason, a lower strength of the dried productis preferable. Since the dried product of the hydrogel particlesproduced by adding an adhesion controlling agent can also controladhesion between water-absorbing resin powders, the strength of thedried product of the hydrogel particles can be lowered. Also, from thisreason, it is preferable to use an adhesion controlling agent.

This adhesion controlling agent is an additive that suppresses excessiveadhesion between particles (also referred to as primary particles) thatconstitute granulated shaped particles. In other words, appropriateadhesion between primary particles occurs to perform granulation. As aresult of appropriate granulation of the primary particles, the adhesioncontrolling agent may be present in the inside of the water-absorbingagent (granulated shaped particles) as well as in the vicinity of thesurface thereof.

Whether the adhesion controlling agent is present in the vicinity of thesurface of the particle or in the inside thereof may be confirmed byanalyzing the cut surface of the water-absorbing agent particle to findout distribution of the adhesion controlling agent or by polishing thewater-absorbing agent particle by a polishing method or a sputteringmethod to find out a change in the amount of the adhesion controllingagent component contained. At this time, as for the number of particlesto be analyzed, it is preferable that 10 or more particles be arbitrarytaken out from particles near the weight average particle diameter andevaluation be carried out with an average value of the analysis value.The inside described herein indicates a portion having a depth of 50 μmor more from the surface of the water-absorbing agent particle.

According to the preferred embodiment of the present invention, theadhesion controlling agent is one or more compounds selected from anonionic substance, an amphoteric substance, an anionic substance, and acationic substance, and the nonionic substance is (a) a polyol, (b) ahydroxy group-modified product of a polyol, (c) side-chain and/orterminal polyether-modified polysiloxane, or (d) an alkylene oxideadduct of higher aliphatic amine, the amphoteric substance is (e)alkylaminobetaine or (f) alkylamine oxide, the anionic substance is (g)a sulfuric acid ester salt of a higher alcohol alkylene oxide adduct or(h) alkyl diphenyl ether disulfonate, and the cationic substance is (i)an ammonium salt. With such a configuration, the desired effect of thepresent invention can be efficiently exerted.

Specific examples of the adhesion controlling agent described in thepresent invention include,

as a nonionic substance,

(a) a polyol,

(b) a hydroxy group-modified product of a polyol,

(c) side-chain and/or terminal polyether-modified polysiloxane, or

(d) an alkylene oxide adduct of higher aliphatic amine,

as an amphoteric substance,

(e) alkylaminobetaine, or

(f) alkylamine oxide

as an anionic substance,

(g) a sulfuric acid ester salt of a higher alcohol alkylene oxideadduct, or

(h) alkyl diphenyl ether disulfonate, and

as a cationic substance,

(i) an ammonium salt.

((a) Polyol)

Examples of a polyol having a plurality of hydroxy groups include (a-1)a non-polymeric polyol and (a-2) a polymeric polyol.

((a-1) Non-Polymeric Polyol)

Specific examples of the non-polymeric polyol having a plurality ofhydroxy groups include di-, tri-, and tetraols such as ethylene glycol,diethylene glycol, triethylene glycol, glycerin, diglycerin,propanediol, butanediol, pentanediol, hexanediol, and octanediol and thelike.

((a-2) Polymeric Polyol)

Specific examples of the polymeric polyol having a plurality of hydroxygroups include polyalkylene glycols such as polyethylene glycol andpolypropylene glycol, and a block copolymer or random copolymer ofpolyethylene glycol and polypropylene glycol. Herein, the number ofcarbon atoms in the repeating unit of the alkylene unit is preferably C1to C6, more preferably C2 to C4, and particularly preferably C2 to C3.(In the present specification, in some cases, the number of carbon atomsis expressed as a numerical value after “C.” For example, when thenumber of carbon atoms is 1, it is expressed as C1, and when the numberof carbon atoms is 10, it is expressed as C10.)

Incidentally, the polyalkylene glycol such as a block copolymer orrandom copolymer of polyethylene glycol and polypropylene glycol can beobtained from the market easily, and for example, the following productsare preferably exemplified.

Products Manufactured by ADEKA CORPORATION

Pluronic Series

Pluronic L-34, Pluronic L-44, Pluronic L-64, Pluronic P-84, PluronicP-85, Pluronic P-103, Pluronic F-68, Pluronic F-88, Pluronic F-108,Pluronic 17R-3, Pluronic 17R-4, Pluronic TR-704, and Pluronic TR-913R

Products manufactured by NOF CORPORATION

PLONON #104, PLONON #204, PLONON #208, UNILUBE 70DP-600B, and UNILUBE70DP-950B

Products manufactured by DKS Co., Ltd.

EPAN 450, EPAN 485, EPAN 680, EPAN 740, EPAN 750, EPAN 785, EPAN U-103,EPAN U-105, and EPAN U-108

According to the preferred embodiment of the present invention, (a) thepolyol is (poly)alkylene glycol. According to such an embodiment,adhesion between crushed hydrogel particles can be controlled.

((b) Hydroxy Group-Modified Product of Polyol)

As for the hydroxy group-modified product of the polyol, it ispreferable that one or more hydroxy groups be modified with an esterand/or an ether. Ether and/or ester modification is preferably ahydrocarbon group, and the hydrocarbon group has preferably C1 to C30,more preferably C2 to C28, still more preferably C3 to C26, particularlypreferably C4 to C24, and most preferably C6 to C22. When the number ofcarbon atoms is more than C30, hydrophobicity may become strong and thesurface tension may be decreased, which is not preferable.

Further, the hydrocarbon group is not limited to a straight chain, andmay be a branched or cyclic saturated hydrocarbon group and/orunsaturated hydrocarbon group, or an aromatic hydrocarbon group such asa phenyl group or an alkylphenyl group. Moreover, the hydrocarbon groupmay have a reactive functional group such as a hydroxy group, an aminogroup, or a glycidyl group.

However, a compound having two or more unsaturated hydrocarbon bonds atstructural ends of the compound is not included. Specifically, adi(meth)acrylate compound having polyethylene glycol at both ends is notincluded. when the adhesion controlling agent is a radicallypolymerizable adhesion controlling agent having two or more unsaturatedbonds at structural ends of the compound, in the case of adding theradically polymerizable adhesion controlling agent in the step (i) orthe step (ii), the radically polymerizable adhesion controlling agentmay be consumed by reaction during polymerization so that the radicallypolymerizable adhesion controlling agent may not remain. Furthermore, inthe case of addition in the step (iii), the radically polymerizableadhesion controlling agent may not be sufficiently consumed to remain ina final product, which causes coloration.

When the adhesion controlling agent has a substituent, such as a hydroxygroup, an amino group, a glycidyl group or the like, this substituenthas reactivity with the carboxyl group included the water-absorbingresin as a functional group. Therefore the adhesion controlling agentremains on the surface of the gel particle so that excessive adhering ofgel during gel-crushing is reduced. Moreover, the functional groupreacts during drying so that crosslinking between gel particles alsooccurs and thus collapse of granulated particles can be furthersuppressed.

Examples of the hydroxy group-modified product of the polyol include(b-1) a glycidyl-modified polyol, (b-2) an alkylene oxide adduct ofhigher alcohol, (b-3) an alkylene oxide adduct of a polyhydric alcoholfatty acid ester.

According to the preferred embodiment of the present invention, (b) thehydroxy group-modified product of the polyol is (b-1) aglycidyl-modified polyol, (b-2) an alkylene oxide adduct of higheralcohol, or (b-3) an alkylene oxide adduct of a polyhydric alcohol fattyacid ester, (b-1) is (poly)alkylene glycol of which at least one of endsis modified with a glycidyl group, (b-2) is (poly)alkylene glycol ofwhich one end is modified with a substituent having a C1 to C30hydrocarbon, and (b-3) is polyhydric alcohol of which at least onehydroxy group is added by alkylene oxide and of which at least onehydroxy group is modified with a substituent having a C1 to C30hydrocarbon via an ester bond, and the polyhydric alcohol is glycerin,pentaerythritol, sorbitol, sorbitan, or sugar. With such aconfiguration, adhesion between crushed hydrogels can be controlled.

((b-1) Glycidyl-Modified Polyol)

The glycidyl-modified polyol is (poly)alkylene glycol of which at leastone of ends is modified with a glycidyl group. Specific examples thereofinclude water-soluble (poly)alkylene glycol diglycidyl ethers such asethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether,and polyethylene glycol diglycidyl ether; and water-soluble polyglycidylethers of polyols such as propylene glycol diglycidyl ether,polypropylene glycol diglycidyl ether, hexanediol diglycidyl ether,glycerol polyglycidyl ether, trimethylolpropane polyglycidyl ether,pentaerythritol polyglycidyl ether, diglycerol polyglycidyl ether,polyglycerol polyglycidyl ether, and sorbitol polyglycidyl ether.

when the adhesion controlling agent is (poly)alkylene glycol of whichtwo or more hydroxy groups are modified with a glycidyl group, in thecase of adding in the step (i) or the step (ii), the adhesioncontrolling agent may be consumed by reaction during polymerization sothat the amount of the adhesion controlling agent may be decreased orthe adhesion controlling agent may not remain during gel-crushing.Therefore, it is preferable to add the adhesion controlling agent afterthe step (ii).

The amount of (b-1) the glycidyl-modified polyol added may be in theabove range of the amount of the adhesion controlling agent added, butis preferably 0.01 wt % to 5 wt %, more preferably 0.02 wt % to 3 wt %,and still more preferably 0.03 wt % to 2 wt % with respect to the rawmaterial monomer of the crosslinked hydrogel polymer.

The glycidyl-modified polyol can be obtained from the market easily, andfor example, the following products are preferably exemplified.

Products Manufactured by Nagase ChemteX Corporation

Denacol EX-145, Denacol EX-171, Denacol EX-211, Denacol EX-212, DenacolEX-252, Denacol EX-810, Denacol EX-811, Denacol EX-850, Denacol EX-851,Denacol EX-821, Denacol EX-830, Denacol EX-832, Denacol EX-841, DenacolEX-861, Denacol EX-911, Denacol EX-941, Denacol EX-920, Denacol EX-931,Denacol EX-313, Denacol EX-314, Denacol EX-321, Denacol EX-411, DenacolEX-421, Denacol EX-512, Denacol EX-521, Denacol EX-612, Denacol EX-614,and Denacol EX-614B

((b-2) Alkylene Oxide Adduct of Higher Alcohol)

The alkylene oxide adduct of higher alcohol is (poly)alkylene glycol ofwhich one end is modified with a substituent having a C1 to C30hydrocarbon, and the general formula thereof is represented in “ChemicalFormula 1.”

As R, the above-described hydrocarbon group having carbon atoms of C1 toC30 is not limited to a straight chain, and may be a branched or cyclicsaturated hydrocarbon group and/or unsaturated hydrocarbon group, anaromatic hydrocarbon group such as an alkylphenyl group or analkylbenzyl group, or a polycyclic aromatic hydrocarbon such as anaphthyl group. Further, the hydrocarbon group may have a reactivefunctional group such as a hydroxy group, an amino group, or a glycidylgroup, or may have an ether bond, an ester bond, a urethane bond, or anamide bond. The number of carbon atoms of the hydrocarbon group ispreferably C1 to C30, more preferably C2 to C28, still more preferablyC3 to C26, particularly preferably C4 to C24, and most preferably C6 toC22. When the hydrocarbon group has more than C30, hydrophobicitybecomes too strong and the surface tension of the water-absorbing agentis significantly decreased, which is not preferable.

AO is a repeating unit that can also be represented by C_(n)H_(2n)O (nis a natural number). The number of carbon atoms thereof is preferablyC1 to C6, more preferably C1 to C3, still more preferably C2 to C3, andparticularly preferably C2, that is, in a case where the repeating unitis CH₂CH₂O that is a structure derived from ethylene oxide addition orethylene glycol condensation, it is particularly preferable.

The repeating unit may be a polymer of units having the same number ofcarbon atoms or may be a block polymer or random polymer of units havingthe different number of carbon atoms.

a is the number of repetitions of repeating units of AO, and ispreferably from 1 to 1,000, more preferably from 2 to 500, and stillmore preferably from 2 to 300. When the number of repetitions of therepeating unit is more than 1,000, the viscosity is increased andaddition is not uniform, which is not preferable.

The HLB of (b-2) the alkylene oxide adduct of higher alcohol as measuredby a Griffin method is preferably 10 to 20, more preferably 12 to 20,and still more preferably 14 to 20. When the HLB is less than the aboverange, hydrophobicity becomes strong so that GCA is reduced, theabsorption speed is decreased, or the surface tension is significantlydecreased, which is not preferable. Further, the upper limit value is 20in the method of determining the HLB.

The amount of (b-2) the alkylene oxide adduct of higher alcohol addedmay be in the above range of the amount of the adhesion controllingagent added, but is preferably 0.01 wt % to 5 wt %, more preferably 0.01wt % to 2 wt %, and still more preferably 0.01 wt % to 0.5 wt % withrespect to the raw material monomer of the crosslinked hydrogel polymer.

(b-2) The alkylene oxide adduct of higher alcohol can be obtained fromthe market easily, and for example, the following products arepreferably exemplified.

Products Manufactured by Kao Corporation

Polyoxyethylene Lauryl Ether

EMULGEN 106 (HLB=10.5), EMULGEN 108 (HLB=12.1), EMULGEN 109P (HLB=13.6),EMULGEN 120 (HLB=15.3), EMULGEN 123P (HLB=16.9), EMULGEN 130K(HLB=18.1), EMULGEN 147 (HLB=16.3), and EMULGEN 150 (HLB=18.4)

Polyoxyethylene Cetyl Ether

EMULGEN 210P (HLB=10.7) and EMULGEN 220 (HLB=14.2)

Polyoxyethylene Stearyl Ether

EMULGEN 320P (HLB=13.9) and EMULGEN 350 (HLB=17.8)

Polyoxyethylene Oleyl Ether

EMULGEN 408 (HLB=10.0), EMULGEN 409PV (HLB=12.0), EMULGEN 420(HLB=13.6), and EMULGEN 430 (HLB=16.2)

Polyoxyethylene Myristyl Ether

EMULGEN 4085 (HLB=18.9)

Polyoxyethylene Octyl Dodecyl Ether

EMULGEN 2020G-HA (HLB=13.0) and EMULGEN 2025G (HLB=15.7)

Products Manufactured by NOF CORPORATION

Polyoxyethylene Isodecyl Ether

Nonion ID-203 (HLB=12.5) and Nonion ID-209 (HLB=14.3)

Polyoxyethylene-2-ethylhexyl Ether

Nonion EH-204 (HLB=11.5) and Nonion EH-208 (HLB=14.6)

Products Manufactured by NIPPON NYUKAZAI CO., LTD.

Polyoxyethylene Nonylphenyl Ether

Newcol 560 (HLB=10.9), Newcol 564 (HLB=12.3), Newcol 565 (HLB=13.3),Newcol 566 (HLB=14.1), Newcol 568 (HLB=15.2), Newcol 504 (HLB=16.0),Newcol 506 (HLB=17.2), Newcol 509 (HLB=18.0), and Newcol 516 (HLB=18.8)

((b-3) Alkylene Oxide Adduct of Polyhydric Alcohol Fatty Acid Ester)

The ethylene oxide adduct of the polyhydric alcohol fatty acid ester ispolyhydric alcohol of which at least one hydroxy group is added alkyleneoxide and of which at least one hydroxy group is modified with asubstituent having a C1 to C30 hydrocarbon via an ester bond. As thepolyhydric alcohol, glycerin, pentaerythritol, sorbitol, sorbitan,sugar, and the like are exemplified.

Preferably, an alkylene oxide adduct of a glycerin fatty acid monoesterand an alkylene oxide adduct of a sorbitan fatty acid monoester areexemplified, and the general formula of the alkylene oxide adduct of theglycerin fatty acid monoester is represented in “Chemical Formula 2.”Since the alkylene oxide adduct of the sorbitan fatty acid monoesterincludes a structural isomer, the alkylene oxide adduct of the sorbitanfatty acid monoester is separately represented in general formulae“Chemical Formula 3” and “Chemical Formula 4.”

As R, the above-described hydrocarbon group having carbon atoms of C1 toC30 is not limited to a straight chain, and may be a branched or cyclicsaturated hydrocarbon group and/or unsaturated hydrocarbon group, anaromatic hydrocarbon group such as an alkylphenyl group or analkylbenzyl group, or a polycyclic aromatic hydrocarbon such as anaphthyl group. Further, the hydrocarbon group may have a reactivefunctional group such as a hydroxy group, an amino group, or a glycidylgroup, or may have an ether bond, an ester bond, a urethane bond, or anamide bond. The number of carbon atoms of the hydrocarbon group ispreferably C1 to C30, more preferably C2 to C28, still more preferablyC3 to C26, particularly preferably C4 to C24, and most preferably C6 toC22. When the hydrocarbon group has more than C30, hydrophobicitybecomes too strong and the surface tension of the water-absorbing agentis significantly decreased, it is not preferable.

A₁O and A₂O are a repeating unit that can also be represented byC_(n)H_(2n)O (n is a natural number). The number of carbon atoms thereofis preferably C1 to C6, more preferably C1 to C3, still more preferablyC2 to C3, and particularly preferably C2, that is, in a case where therepeating unit is CH₂CH₂O that is a structure derived from ethyleneoxide addition or ethylene glycol condensation, it is particularlypreferable.

The repeating unit may be a polymer of units having the same number ofcarbon atoms or may be a block polymer or random polymer of units havingthe different number of carbon atoms. A₁O and A₂O may be different fromeach other or may be the same.

a and b are the number of repetitions of repeating units of A₁O and A₂O,and the total of a+b is preferably from 1 to 1,000, more preferably from2 to 500, and still more preferably from 2 to 300. a and b may bedifferent from each other or may be the same. When the total of a+b ismore than 1,000, the viscosity is increased and addition is not uniform,which is not preferable.

As R, the above-described hydrocarbon group having carbon atoms of C1 toC30 is not limited to a straight chain, and may be a branched or cyclicsaturated hydrocarbon group and/or unsaturated hydrocarbon group, anaromatic hydrocarbon group such as an alkylphenyl group or analkylbenzyl group, or a polycyclic aromatic hydrocarbon such as anaphthyl group. Further, the hydrocarbon group may have a reactivefunctional group such as a hydroxy group, an amino group, or a glycidylgroup, or may have an ether bond, an ester bond, a urethane bond, or anamide bond. The number of carbon atoms of the hydrocarbon group ispreferably C1 to C30, more preferably C2 to C28, still more preferablyC3 to C26, particularly preferably C4 to C24, and most preferably C6 toC22. When the hydrocarbon group has more than C30, hydrophobicitybecomes too strong and the surface tension of the water-absorbing agentis significantly decreased, it is not preferable.

A₁O, A₂O, and A₃O are a repeating unit that can also be represented byC_(n)H_(2n)O (n is a natural number). The number of carbon atoms thereofis preferably C1 to C6, more preferably C1 to C3, still more preferablyC2 to C3, and particularly preferably C2, that is, in a case where therepeating unit is CH₂CH₂O that is a structure derived from ethyleneoxide addition or ethylene glycol condensation, it is a repeating unitis particularly preferable.

The repeating unit may be a polymer of units having the same number ofcarbon atoms or may be a block polymer or random polymer of units havingthe different number of carbon atoms. A₁O, A₂O, and A₃O may be differentfrom each other or may be the same.

a, b, and c are the number of repetitions of the repeating units, andthe total of a+b+c is preferably from 1 to 1,000, more preferably from 2to 500, and still more preferably from 2 to 300. a and b may bedifferent from each other or may be the same. When the total of a+b+c ismore than 1,000, the viscosity is increased and addition is not uniform,it is not preferable.

a, b, and c are an average repeating unit of polyethylene glycol, andthe total amount of a+b+c is preferably from 1 to 300, more preferablyfrom 2 to 200, and still more preferably from 2 to 100. a, b, and c maybe different from each other or may be the same.

The HLB of (b-3) the alkylene oxide adduct of the polyhydric alcoholfatty acid ester as measured by a Griffin method is preferably 10 to 20,more preferably 12 to 20, and still more preferably 14 to 20. When theHLB is less than the above range, hydrophobicity becomes strong so thatGCA is reduced, the absorption speed is decreased, or the surfacetension is significantly decreased, it is not preferable. Further, theupper limit value is 20 in the method of determining the HLB.

The amount of (b-3) the alkylene oxide adduct of a polyhydric alcoholfatty acid ester added may be in the above range of the amount of theadhesion controlling agent added, but is preferably 0.01 wt % to 5 wt %,more preferably 0.01 wt % to 2 wt %, and still more preferably 0.01 wt %to 0.5 wt % with respect to the raw material monomer of the crosslinkedhydrogel polymer.

(b-3) The alkylene oxide adduct of a polyhydric alcohol fatty acid estercan be obtained from the market easily, and for example, the followingproducts are preferably exemplified.

Products Manufactured by Kao Corporation

Polyoxyethylene Sorbitan Monolaurate

RHEODOL TW-L120 (HLB=16.7), RHEODOL TW-L106 (HLB=13.3), and RHEODOLSUPER TW-L120

Polyoxyethylene Sorbitan Monopalmitate

RHEODOL TW-P120 (HLB=15.6)

Polyoxyethylene Sorbitan Monostearate

RHEODOL TW-S120V (HLB=14.9)

Polyoxyethylene Sorbitan Tristearate

RHEODOL TW-S320V (HLB=10.5)

Polyoxyethylene Sorbitan Monooleate

RHEODOL TW-0120V (HLB=15.0) and RHEODOL TW-0106V (HLB=10.0)

Polyoxyethylene Sorbitan Trioleate

RHEODOL TW-0320V (HLB=11.0)

Products Manufactured by NOF CORPORATION

Polyoxyethylene Glyceryl Coconut Oil Fatty Acid

UNIGLY MK-207 (HLB=13.0) and UNIGLY MK-230 (HLB=17.4)

((c) Side-Chain and/or Terminal Polyether-Modified Polysiloxane)

The polyether-modified site in the polysiloxane is not particularlylimited, but may be a side chain of the polysiloxane, both ends of thepolysiloxane, one end of the polysiloxane, or both of a side chain andboth ends of the polysiloxane. Examples of a polyether-modified groupinclude a polyoxyethylene group, a polyoxypropylene group, and a grouphaving both of a polyoxyethylene group and a polyoxypropylene group.

The HLB of the polyether-modified polysiloxane as measured by a Griffinmethod is preferably 10 to 20, more preferably 12 to 20, and still morepreferably 14 to 20. When the HLB is less than the above range,hydrophobicity becomes strong so that GCA is reduced, the absorptionspeed is decreased, or the surface tension is significantly decreased,it is not preferable. Further, the upper limit value is 20 in the methodof determining the HLB.

The amount of (c) the side-chain and/or terminal polyether-modifiedpolysiloxane added may be in the above range of the amount of theadhesion controlling agent added, but is preferably 0.01 wt % to 5 wt %,more preferably 0.01 wt % to 2 wt %, and still more preferably 0.01 wt %to 0.5 wt % with respect to the raw material monomer of the crosslinkedhydrogel polymer.

The polyether-modified siloxane can be obtained from the market easily,and for example, the following products are preferably exemplified.

Products Manufactured by Shin-Etsu Chemical Co., Ltd.

KF-351A (HLB=12), KF-353 (HLB=10), KF-354L (HLB=16), KF-355A (HLB=12),KF-615A (HLB=10), KF-640 (HLB=14), KF-642 (HLB=12), KF-643 (HLB=14), andKF-6011 (HLB=12)

Products Manufactured by Dow Corning Toray Co., Ltd.

FZ-77 (HLB=11) and L-7604 (HLB=11)

((d) Alkylene Oxide Adduct of Higher Aliphatic Amine)

According to the preferred embodiment of the present invention, (d) thealkylene oxide adduct of higher aliphatic amine is primary amine havinga C1 to C30 hydrocarbon of which two hydrogens of the primary amine areadded by alkylene oxide. With such a configuration, adhesion betweencrushed hydrogels can be controlled.

The alkylene oxide adduct of higher aliphatic amine is primary aminehaving a C1 to C30 hydrocarbon group of which two hydrogens are modifiedwith (poly)alkylene glycol, and the general formula thereof isrepresented in “Chemical Formula 5.”

As R, the above-described hydrocarbon group having carbon atoms of C1 toC30 is not limited to a straight chain, and may be a branched or cyclicsaturated hydrocarbon group and/or unsaturated hydrocarbon group, anaromatic hydrocarbon group such as an alkylphenyl group or analkylbenzyl group, or a polycyclic aromatic hydrocarbon such as anaphthyl group. Further, the hydrocarbon group may have a reactivefunctional group such as a hydroxy group, an amino group, or a glycidylgroup, or may have an ether bond, an ester bond, a urethane bond, or anamide bond. The number of carbon atoms of the hydrocarbon group ispreferably C1 to C30, more preferably C2 to C28, still more preferablyC3 to C26, particularly preferably C4 to C24, and most preferably C6 toC22. When the hydrocarbon group has more than C30, hydrophobicitybecomes too strong and the surface tension of the water-absorbing agentis significantly decreased, which is not preferable.

A₁O and A₂O are a repeating unit that can also be represented byC_(n)H_(2n)O (n is a natural number). The number of carbon atoms thereofis preferably C1 to C6, more preferably C1 to C3, still more preferablyC2 to C3, and particularly preferably C2, that is, a case where CH₂CH₂Othat is a structure derived from ethylene oxide addition or ethyleneglycol condensation is a repeating unit is particularly preferable. Thealkylene unit may be a polymer of units having the same number of carbonatoms or may be a block polymer or random polymer of units having thedifferent number of carbon atoms. A₁O and A₂O may be different from eachother or may be the same.

a and b are a repeating unit of an alkylene glycol unit, and the totalof a +b is preferably from 1 to 1,000, more preferably from 2 to 500,and still more preferably from 2 to 300. a and b may be different fromeach other or may be the same. When the total of a +b is more than1,000, the viscosity is increased and addition is not uniform, it is notpreferable.

The HLB of (d) the alkylene oxide adduct of higher aliphatic amine asmeasured by a Griffin method is preferably 10 to 20, more preferably 12to 20, and still more preferably 14 to 20. When the HLB is less than theabove range, hydrophobicity becomes strong so that GCA is reduced, theabsorption speed is decreased, or the surface tension is significantlydecreased, it is not preferable. Further, the upper limit value is 20 inthe method of determining the HLB.

The amount of (d) the alkylene oxide adduct of higher aliphatic amineadded may be in the above range of the amount of the adhesioncontrolling agent added, but is preferably 0.01 wt % to 5 wt %, morepreferably 0.01 wt % to 2 wt %, and still more preferably 0.01 wt % to0.5 wt % with respect to the raw material monomer of the crosslinkedhydrogel polymer.

(d) The alkylene oxide adduct of higher aliphatic amine can be obtainedfrom the market easily, and for example, the following products arepreferably exemplified.

Products Manufactured by NOF CORPORATION

Polyoxyethylene Lauryl Amine

NYMEEN L-207 (HLB=12.5)

Polyoxyethylene Alkyl(Coconut) Amine

NYMEEN F-215 (HLB=15.4)

Polyoxyethylene Stearylamine

NYMEEN S-210 (HLB=12.5), NYMEEN S-215 (HLB=14.5), and NYMEEN S-220(HLB=15.4)

Polyoxyethylene Beef Tallow Alkyl Amine

NYMEEN T2-210 (HLB=12.5) and NYMEEN T2-230 (HLB=16.7)

Polyoxyethylene Alkyl Propylene Diamine

NYMEEN DT-208 (HLB=10.7)

Products Manufactured by Kao Corporation

AMIET 105A (HLB=10.8), AMIET 320 (HLB=15.4)

((e) Alkylaminobetaine)

The alkylaminobetaine has a cationic group and an anionic group atpositions which are not adjacent in the same molecule, the cationicgroup is secondary to quaternary ammoniums, at least one of secondary toquaternary ammoniums is modified with a substituent having a C1 to C30hydrocarbon group, and the general formula thereof is represented in“Chemical Formula 6.”

As R₁, the above-described hydrocarbon group having carbon atoms of C1to C30 is not limited to a straight chain, and may be a branched orcyclic saturated hydrocarbon group and/or unsaturated hydrocarbon group,an aromatic hydrocarbon group such as an alkylphenyl group or analkylbenzyl group, or a polycyclic aromatic hydrocarbon such as anaphthyl group. Further, the hydrocarbon group may have a reactivefunctional group such as a hydroxy group, an amino group, or a glycidylgroup, or may have an ether bond, an ester bond, a urethane bond, or anamide bond. The number of carbon atoms of the hydrocarbon group ispreferably C1 to C30, more preferably C2 to C28, still more preferablyC3 to C26, particularly preferably C4 to C24, and most preferably C6 toC22. When the hydrocarbon group has more than C30, hydrophobicitybecomes too strong and the surface tension of the water-absorbing agentis significantly decreased, which is not preferable.

R₂ and R₃ are hydrogen or a hydrocarbon group, and the above-describedhydrocarbon group having carbon atoms of C1 to C30 is not limited to astraight chain, and may be a branched or cyclic saturated hydrocarbongroup and/or unsaturated hydrocarbon group, an aromatic hydrocarbongroup such as an alkylphenyl group or an alkylbenzyl group, or apolycyclic aromatic hydrocarbon such as a naphthyl group. Further, thehydrocarbon group may have a reactive functional group such as a hydroxygroup, an amino group, or a glycidyl group, or may have an ether bond,an ester bond, a urethane bond, or an amide bond. The number of carbonatoms of the hydrocarbon group is preferably C1 to C30, more preferablyC1 to C25, and still more preferably C1 to C20. When the hydrocarbongroup has more than C30, hydrophobicity becomes too strong and thesurface tension of the water-absorbing agent is significantly decreased,which is not preferable. R₁, R₂, and R₃ may be different from each otheror may be the same.

The structure of X is not particularly limited except that X has carbonatoms of C1 or more.

Examples of an anionic portion (Z) include carboxylates, sulfonates, andphosphates.

However, in addition to the matter that is represented in the generalformual “Chemical Formula 6,” the alkylaminobetaine has a cationic groupon an imidazolium ring like AMPHITOL 20YB (manufactured by KaoCorporation) represented in the following “Chemical Formula 7.”

As R₁, the above-described hydrocarbon group having carbon atoms of C1to C30 is not limited to a straight chain, and may be a branched orcyclic saturated hydrocarbon group and/or unsaturated hydrocarbon group,an aromatic hydrocarbon group such as an alkylphenyl group or analkylbenzyl group, or a polycyclic aromatic hydrocarbon such as anaphthyl group. Further, the hydrocarbon group may have a reactivefunctional group such as a hydroxy group, an amino group, or a glycidylgroup, or may have an ether bond, an ester bond, a urethane bond, anamide bond. The number of carbon atoms of the hydrocarbon group ispreferably C1 to C30, more preferably C2 to C28, still more preferablyC3 to C26, particularly preferably C4 to C24, and most preferably C6 toC22. When the hydrocarbon group has more than C30, hydrophobicitybecomes too strong and the surface tension of the water-absorbing agentis significantly decreased, which is not preferable.

R₂ is hydrogen or a hydrocarbon group, and the above-describedhydrocarbon group having carbon atoms of C1 to C30 is not limited to astraight chain, and may be a branched or cyclic saturated hydrocarbongroup and/or unsaturated hydrocarbon group, an aromatic hydrocarbongroup such as an alkylphenyl group or an alkylbenzyl group, or apolycyclic aromatic hydrocarbon such as a naphthyl group. Further, thehydrocarbon group may have a reactive functional group such as a hydroxygroup, an amino group, or a glycidyl group, or may have an ether bond,an ester bond, a urethane bond, or an amide bond. The number of carbonatoms of the hydrocarbon group is preferably C1 to C30, more preferablyC1 to C25, and still more preferably C1 to C20. When the hydrocarbongroup has more than C30, hydrophobicity becomes too strong and thesurface tension of the water-absorbing agent is significantly decreased,it is not preferable. R₁ and R₂ may be different from each other or maybe the same.

The structure of X is not particularly limited except that X has carbonatoms of C1 or more.

Examples of an anionic portion (Z) include carboxylates, sulfonates, andphosphates.

(f) Alkylamine Oxide

The alkylamine oxide has a cationic group and an anionic group atpositions which are not adjacentin the same molecule, the cationic groupis secondary to quaternary ammoniums, secondary to quaternary ammoniumsis modified with at least one of a substituent having a C1 to C30hydrocarbon group, and the general formula thereof is represented in“Chemical Formula 8.”

As R₁, the above-described hydrocarbon group having carbon atoms of C1to C30 is not limited to a straight chain, and may be a branched orcyclic saturated hydrocarbon group and/or unsaturated hydrocarbon group,an aromatic hydrocarbon group such as an alkylphenyl group or analkylbenzyl group, or a polycyclic aromatic hydrocarbon such as anaphthyl group. Further, the hydrocarbon group may have a reactivefunctional group such as a hydroxy group, an amino group, or a glycidylgroup, or may have an ether bond, an ester bond, a urethane bond, or anamide bond. The number of carbon atoms of the hydrocarbon group ispreferably C1 to C30, more preferably C2 to C28, still more preferablyC3 to C26, particularly preferably C4 to

C24, and most preferably C6 to C22. When the hydrocarbon group has morethan C30, hydrophobicity becomes too strong and the surface tension ofthe water-absorbing agent is significantly decreased, which is notpreferable.

R₂ and R₃ are hydrogen or a hydrocarbon group, and the above-describedhydrocarbon group having carbon atoms of C1 to C30 is not limited to astraight chain, and may be a branched or cyclic saturated hydrocarbongroup and/or unsaturated hydrocarbon group, an aromatic hydrocarbongroup such as an alkylphenyl group or an alkylbenzyl group, or apolycyclic aromatic hydrocarbon such as a naphthyl group. Further, thehydrocarbon group may have a reactive functional group such as a hydroxygroup, an amino group, or a glycidyl group, or may have an ether bond,an ester bond, a urethane bond, or an amide bond. The number of carbonatoms of the hydrocarbon group is preferably C1 to C30, more preferablyC1 to C25, and still more preferably C1 to C20. When the hydrocarbongroup has more than C30, hydrophobicity becomes too strong and thesurface tension of the water-absorbing agent is significantly decreased,which is not preferable. R₁, R₂, and R₃ may be different from each otheror may be the same.

The amounts of (e) the alkylaminobetaine and (f) the alkylamine oxideadded may be in the above range of the amount of the adhesioncontrolling agent added, but are preferably 0.01 wt % to 5 wt %, morepreferably 0.01 wt % to 2 wt %, and still more preferably 0.01 wt % to0.5 wt % with respect to the raw material monomer of the crosslinkedhydrogel polymer.

(e) The alkylaminobetaine and (f) the alkylamine oxide can be obtainedfrom the market easily, and for example, the following products arepreferably exemplified.

Products Manufactured by Kao Corporation:

AMPHITOL 20BS, AMPHITOL 24B (desalinated product of 20BS), AMPHITOL 86B,AMPHITOL 20N, AMPHITOL 20YB, AMPHITOL 20AB, AMPHITOL 55AB, and AMPHITOL20HD

Products Manufactured by DKS Co., Ltd.:

AMOGEN S-H, AMOGEN K, AMOGEN LB-C, AMOGEN CB-H, AMOGEN HB-C, and AMOGENAOL

Products Manufactured by ADEKA CORPORATION:

ADEKA AMPHOTE PB-30L and ADEKA AMPHOTE AB-35L

Products Manufactured by NOF CORPORATION:

NISSANANON BF, NISSANANON BL, NISSANANON BL-SF, NISSANANON BDF-R,NISSANANON BDF-SF, NISSANANON BDC-SF, NISSANANON BDL-SF, NISSANANONGLM-R, UNISAFE A-LM, UNISAFE A-SM, and UNISAFE A-LE

Products Manufactured by NIPPON NYUKAZAI CO., LTD.:

Texnol R2

((g) Sulfuric Acid Ester Salt of Higher Alcohol Ethylene Oxide Adduct)

According to the preferred embodiment of the present invention, (g) thesulfuric acid ester salt of the higher alcohol alkylene adduct is(poly)alkylene glycol of which one end is modified with a substituenthaving a C1 to C30 hydrocarbon and the other end is a sulfuric acidester salt. With such a configuration, adhesion between crushedhydrogels can be controlled.

The sulfuric acid ester salt of the higher alcohol ethyleneoxide adductis (poly)alkylene glycol of which one end is modified with a substituenthaving a C1 to C30 hydrocarbon and the other end is a sulfuric acidester salt, and the general formula thereof is represented in “ChemicalFormula 9.”

As R, the above-described hydrocarbon group having carbon atoms of C1 toC30 is not limited to a straight chain, and may be a branched or cyclicsaturated hydrocarbon group and/or unsaturated hydrocarbon group, anaromatic hydrocarbon group such as an alkylphenyl group or analkylbenzyl group, or a polycyclic aromatic hydrocarbon such as anaphthyl group. Further, the hydrocarbon group may have a reactivefunctional group such as a hydroxy group, an amino group, or a glycidylgroup, or may have an ether bond, an ester bond, a urethane bond, or anamide bond. The number of carbon atoms of the hydrocarbon group ispreferably C1 to C30, more preferably C2 to C28, still more preferablyC3 to C26, particularly preferably C4 to C24, and most preferably C6 toC22. When the hydrocarbon group has more than C30, hydrophobicitybecomes too strong and the surface tension of the water-absorbing agentis significantly decreased, which is not preferable.

AO is a repeating unit that can also be represented by C_(n)H_(2n)O (nis a natural number). The number of carbon atoms thereof is preferablyC1 to C6, more preferably C1 to C3, still more preferably C2 to C3, andparticularly preferably C2, that is, in a case where the repeating unitis CH₂CH₂O that is a structure derived from ethylene oxide addition orethylene glycol condensation, it is a repeating unit is particularlypreferable.

The repeating unit may be a polymer of units having the same number ofcarbon atoms or may be a block polymer or random polymer of units havingthe different number of carbon atoms.

a is the number of repetitions of repeating units of AO, and ispreferably from 1 to 1,000, more preferably from 2 to 500, and stillmore preferably from 2 to 300. When the number of repetitions of therepeating unit is more than 1,000, the viscosity is increased andaddition is not uniform, which is not preferable.

As M, alkali metals (Li, Na, and K) and ammonium ions are exemplified.

The amount of (g) the sulfuric acid ester salt of the higher alcoholalkylene oxide adduct added may be in the above range of the amount ofthe adhesion controlling agent added, but is preferably 0.01 wt % to 5wt %, more preferably 0.01 wt % to 2 wt %, and still more preferably0.01 wt % to 0.5 wt % with respect to the raw material monomer of thecrosslinked hydrogel polymer.

(g) The sulfuric acid ester salt of the higher alcohol alkylene oxideadduct can be obtained from the market easily, and for example, thefollowing products are preferably exemplified.

Products Manufactured by Kao Corporation

Sodium Polyoxyethylene Lauryl Ether Sulfate

EMAL 20C, EMAL E-27C, EMAL 270J, and EMAL 20CM

Products Manufactured by NIPPON NYUKAZAI CO., LTD.

Polyoxyethylene Alkyl Ether Sulfuric Acid Ester Salt

Newcol 1020-SN, Newcol 2308-SF, Newcol 2320-SN, Newcol 2360-SN, Newcol1305-SN, Newcol 1330-SF, Newcol 1703-SFD, and Newcol 1525-SFC

Products Manufactured by NOF CORPORATION

Polyoxyethylene Alkyl Ether Sulfate Sodium

PERSOFT EP, NISSAN TRAX K-40, NISSAN TRAX K-300, PERSOFT EF, PERSOFTEDO, PERSOFT EL, amd PERSOFT EK

((h) Alkyl Diphenyl Ether Disulfonate)

The general formula of the alkyl diphenyl ether disulfonate isrepresented in “Chemical Formula 10.”

As R, the above-described hydrocarbon group having carbon atoms of C1 toC30 is not limited to a straight chain, and may be a branched or cyclicsaturated hydrocarbon group and/or unsaturated hydrocarbon group, anaromatic hydrocarbon group such as an alkylphenyl group or analkylbenzyl group, or a polycyclic aromatic hydrocarbon such as anaphthyl group. Further, the hydrocarbon group may have a reactivefunctional group such as a hydroxy group, an amino group, or a glycidylgroup, or may have an ether bond, an ester bond, a urethane bond, or anamide bond. The number of carbon atoms of the hydrocarbon group ispreferably C1 to C30, more preferably C2 to C28, still more preferablyC3 to C26, particularly preferably C4 to C24, and most preferably C6 toC22. When the hydrocarbon group has more than C30, hydrophobicitybecomes too strong and the surface tension of the water-absorbing agentis significantly decreased, which is not preferable.

As M, alkali metals (Li, Na, and K) and ammonium ions are exemplified.

The amount of (h) the alkyl diphenyl ether disulfonate added may be inthe above range of the amount of the adhesion controlling agent added,but is preferably 0.01 wt % to 5 wt %, more preferably 0.01 wt % to 2 wt%, and still more preferably 0.01 wt % to 0.5 wt % with respect to theraw material monomer of the crosslinked hydrogel polymer.

(h) The alkyl diphenyl ether disulfonate can be obtained from the marketeasily, and for example, the following products are preferablyexemplified.

Products Manufactured by Kao Corporation

Sodium Alkyl Diphenyl Ether Disulfonate

PELEX SS-L and PELEX SS-H

Products Manufactured by TAKEMOTO OIL & FAT Co., Ltd.

Sodium Alkyl Diphenyl Ether Disulfonate

PIONIN A-43-D and TAKESURF A-43-NQ

((i) Ammonium Salt)

The ammonium salt is ammonium salt of which at least one hydrogen ismodified with a substituent having a C1 to C30 hydrocarbon, and thegeneral formula thereof is represented in “Chemical Formula 11.”

As R₁, the above-described hydrocarbon group having carbon atoms of C1to C30 is not limited to a straight chain, and may be a branched orcyclic saturated hydrocarbon group and/or unsaturated hydrocarbon group,an aromatic hydrocarbon group such as an alkylphenyl group or analkylbenzyl group, or a polycyclic aromatic hydrocarbon such as anaphthyl group. Further, the hydrocarbon group may have a reactivefunctional group such as a hydroxy group, an amino group, or a glycidylgroup, or may have an ether bond, an ester bond, a urethane bond, or anamide bond. The number of carbon atoms of the hydrocarbon group ispreferably C1 to C30, more preferably C2 to C28, still more preferablyC3 to C26, particularly preferably C4 to C24, and most preferably C6 toC22. When the hydrocarbon group has more than C30, hydrophobicitybecomes too strong and the surface tension of the water-absorbing agentis significantly decreased, which is not preferable.

R₂, R₃, and R₄ are hydrogen or a hydrocarbon group, and theabove-described hydrocarbon group having carbon atoms of C1 to C30 isnot limited to a straight chain, and may be a branched or cyclicsaturated hydrocarbon group and/or unsaturated hydrocarbon group, anaromatic hydrocarbon group such as an alkylphenyl group or analkylbenzyl group, or a polycyclic aromatic hydrocarbon such as anaphthyl group. Further, the hydrocarbon group may have a reactivefunctional group such as a hydroxy group, an amino group, or a glycidylgroup, or may have an ether bond, an ester bond, a urethane bond, or anamide bond. The number of carbon atoms of the hydrocarbon group ispreferably C1 to C30, more preferably C1 to C25, and still morepreferably C1 to C20. When the hydrocarbon group has more than C30,hydrophobicity becomes too strong and the surface tension of thewater-absorbing agent is significantly decreased, which is notpreferable. R₁, R₂, R₃, and R₄ may be different from each other or maybe the same.

N is counter anion of ammonium cation, and examples thereof includehalogen ion, carboxylate ion, sulfonate ion, hydroxy ion, BF⁴⁻, PF⁶⁻,ClO⁴⁻, AsF⁶⁻, and SbF⁶⁻.

The amount of (i) the ammonium salt added may be in the above range ofthe amount of the adhesion controlling agent added, but is preferably0.01 wt % to 5 wt %, more preferably 0.01 wt % to 2 wt %, and still morepreferably 0.01 wt % to 0.5 wt % with respect to the raw materialmonomer of the crosslinked hydrogel polymer.

(i) The ammonium salt can be obtained from the market easily, and forexample, the following products are preferably exemplified.

Products Manufactured by Kao Corporation

Coconut Amine Acetate

ACETAMIN 24

Stearylamine Acetate

ACETAMIN 86

Lauryl Trimethyl Ammonium Chloride

QUARTAMIN 24P

Stearyl Trimethyl Ammonium Chloride

QUARTAMIN 86W

Cetyl Trimethyl Ammonium Chloride

QUARTAMIN 60W

Distearyl Dimethyl Ammonium Chloride

QUARTAMIN D86P

Alkylbenzyl Dimethyl Ammonium Chloride

SANISOL C and SANISOL B-50

Products Manufactured by NOF CORPORATION

Tetradecyl Amine Acetate

NISSANCATION MA

Dodecyl Trimethyl Ammonium Chloride

NISSANCATION BB

Coco Alkyl Trimethyl Ammonium Chloride

NISSANCATION FB

Hexadecyl Trimethyl Ammonium Chloride

NISSANCATION PB-300

Beef-Tallow-Alkyl Trimethyl Ammonium Chloride

NISSANCATION ABT2-500

Octadecyl Trimethyl Ammonium Chloride

NISSANCATION AB and NISSANCATION AB-600

Behenyl Trimethyl Ammonium Chloride

NISSANCATION VB-M Flake and NISSANCATION VB-F

Didecyl Dimethyl Ammonium Chloride

NISSANCATION 2-DB-500E

Dioleyl Dimethyl Ammonium Chloride

NISSANCATION 2-OLR

Coco Alkyl Dimethyl Benzyl Ammonium Chloride

NISSANCATION F2-50R

Tetradecyl Dimethyl Benzyl Ammonium Chloride

NISSANCATION M2-100R

The adhesion controlling agent used in the present applicationpreferably has a hydrophilic unit (a cationic group such as a quaternaryammonium salt, sulfonate, amine, or polyethylene glycol chain) and ahydrophobic unit (a hydrocarbon group) in the same compound. As thehydrophilic unit, a quaternary ammonium salt and a polyethylene glycolchain are particularly preferable.

It is considered that the hydrophilic unit of the adhesion controllingagent interacts with the inside or/and the surface of the hydrophilicparticulate absorbing agent so as to be less likely to be eluted fromthe absorbing agent. For this reason, it is considered that a decreasein the surface tension of the absorbing agent is suppressed and there-wet of liquid from a water-absorbing material is suppressed accordingto the decrease in the surface tension.

It is considered that the adhesion controlling agent is gathered on thesurface layer (a portion close to an air layer) of the hydrogelparticles after gel-crushing because of its hydrophobic property.So thehydrophobic unit of the adhesion controlling agent re-adhesion betweengel particles can be suppressed since the effect of suppressing adhesionbetween the crushed gels is enhanced. Meanwhile, it is considered thatthe hydrophilic unit interacts with the gel so as to serve as an anchorfor causing the adhesion controlling agent to remain on the surface ofthe gel particles.

Further, the adhesion controlling agent used in the present applicationis also preferably polyethylene glycol having only a hydrophilic unitwithout a hydrophobic unit. Particularly, it is considered that thepolyethylene glycol having only a hydrophilic unit interacts with theinside or/and the surface of the hydrophilic particulate absorbing agentso as to be particularly less likely to be eluted from the absorbingagent. Further, since the optimal value range of the addition amount iswide and it is insensitive to a variation in the addition amount, theoperation control range in production can be wide, which is preferable.Furthermore, when the polyethylene glycol is present on the surface ofthe hydrogel particles, a carboxyl group that is a functional group ofthe water-absorbing resin reacts with hydroxy groups at both ends of thepolyethylene glycol in the drying step so as to crosslink primaryparticles that constitute granulated shaped particles, and thus theeffect of suppressing collapse of the granulated shaped particles isachieved when the granulated shaped particles absorb water to beswollen, which is preferable.

As a main effect obtained by using an adhesion controlling agent,controlling adhesion between primary particles of the hydrogel particlesis exemplified. Further, as an adventitious effect, improvingflowability of the hydrogel and suppressing the strength of the driedproduct of the hydrogel particles are exemplified.

Further, when an adhesion controlling agent is used, it is possible toreduce a load to a gel-crusher by the lubrication effect of the adhesioncontrolling agent, productivity is further increased, and degradation ofthe gel during gel-crushing is suppressed, which is preferable.

The HLB of the nonionic substance as measured by a Griffin method ispreferably 10 to 20, more preferably 12 to 20, and still more preferably14 to 20. When the HLB is less than the above range, hydrophobicitybecomes strong so that GCA is reduced, the absorption speed isdecreased, or the surface tension is significantly decreased, which isnot preferable. Further, the upper limit value is 20 in the method ofdetermining the HLB.

The molecular weight (weight average molecular weight) of the adhesioncontrolling agent is not particularly limited. However, in order toexert the effect with a smaller addition amount or to avoid negativeeffect such as a decrease in fluid retention capacity, the molecularweight (hereinafter, the weight average molecular weight in a case wherethe adhesion controlling agent is a polymer) is preferably in a range of100 to 1,000,000, more preferably in a range of 150 to 500,000, stillmore preferably in a range of 200 to 500,000 or 300 to 300,000, stillmore preferably in a range of 500 to 200,000, particularly preferably ina range of 1,000 to 50,000 or less, and most preferably in a range of30,000 or less. Further, particularly, in a case where polyethyleneglycol is used as an adhesion controlling agent, when the weight averagemolecular weight is 500 or more, the desired effect of the presentinvention can be efficiently exerted.

When the molecular weight is less than the above range, the adhesioncontrolling agent is easily volatilized and the effect thereof isdecreased, which is not preferable. When the molecular weight is morethan the above range, the viscosity is increased and addition is notuniform, which is not preferable.

According to the preferred embodiment of the present invention, weightaverage molecular weights of the nonionic substance and (g) the sulfuricacid ester salt of the higher alcohol ethylene oxide adduct are eachindependently 200 to 200,000. According to such an embodiment, theadhesion between crushed hydrogel particles can be controlled. Further,in this embodiment, the weight average molecular weight is particularlypreferably 50,000 or less and most preferably 30,000 or less.

(2-4) Drying Step (Step (iv))

This is a step for drying the hydrogel (hydrogel particles) obtainedthrough the polymerization step and the like to obtain a dry polymer(dried product). Incidentally, in a case where the polymerization stepis aqueous solution polymerization, gel-crushing (grain refining) isperformed before drying the hydrogel. Further, the dry polymer(agglomerate) (dried product) obtained in the drying step may besupplied to the pulverizing step as it is.

The drying method in the present invention is not particularly limited,and various methods can be employed. Specific examples thereof includeheat drying, hot air drying, drying under reduced pressure , infrareddrying, microwave drying, drying by azeotropic dehydration with ahydrophobic organic solvent, and high humidity drying using hot watervapor or the like. One kind of these can be used, or two kinds of thesecan be used together.

By performing drying in this drying step, finely crushed particulatehydrogels adhere to each other to form particles having a granulatedform.

Incidentally, granulation in the present invention means formation of aparticle larger than the original particle (primary particle) byattaching the particles together by a physical or chemical method, andit is characterized that the adhesion of the primary particles is looseor in contact with each other in point. A granulated product orgranulated particles of the present invention are particles adhering toeach other in a state where primary particles are clearly identified asshown in the drawings of Examples. Incidentally, the degree of adhesionis controlled by existence and types of the adhesion controlling agentso that GCA, FGBP, vortex, AAP, and the like can be controlled.

The drying temperature in the present invention is preferably 100° C. to300° C. and more preferably 150° C. to 250° C. Further, the drying timedepends on the surface area and the moisture content of the hydrogelparticles, the kind of a drier, and the like, and thus is, for example,preferably 1 minute to 5 hours and more preferably 5 minutes to 1 hour.

Furthermore, the solids content of the water-absorbing resin (driedproduct) determined from an amount lost from drying (1 g of powder orparticles is dried at 180° C. for 3 hours) is preferably 80 wt % ormore, more preferably 85 wt % to 99 wt %, still more preferably 90 wt %to 98 wt %, and particularly preferably 92 wt % to 97 wt %.

(2-5) Pulverizing and Classification Step (Step (v))

This is a step for pulverizing and/or classifying the dry polymer (driedproduct) obtained in the drying step to preferably obtainwater-absorbing resin powder having a specific particle size.Incidentally, this step is different from the above (2-3) Gel-CrushingStep in that the object to be crushed has passed through the dryingstep.

This step is performed before and/or after (2-6) Surface CrosslinkingStep, is preferably performed before (2-6) Surface Crosslinking Step,and is more preferably performed at least two times before and after(2-6) Surface Crosslinking Step.

Examples of a device (pulverizer) used in the pulverizing step in thepresent invention include a high-speed rotary pulverizer such as a rollmill, a hammer mill, a screw mill, or a pin mill, a vibration mill, aknuckle type pulverizer, and a cylindrical mixer or the like. Thesedevices are used together, if necessary.

(Particle Size)

The weight average particle diameter (D50) of the water-absorbing resinpowder before surface crosslinking is preferably 300 μm to 500 μm, morepreferably 310 μm to 480 μm, and still more preferably 320 μm to 450 μmfrom the viewpoint of handleability (particularly, handleability undermoisture absorption), GCA, FGBP, the water absorption speed, the fluidretention capacity under pressure, and the like.

Further, a smaller amount of fine particles, which have a particlediameter of less than 150 μm specified by standard sieve classification,contained is better. The amount is preferably 0 wt % to 5 wt %, morepreferably 0 wt % to 3 wt %, and still more preferably 0 wt % to 2 wt %with respect to the entire water-absorbing resin powder.

Furthermore, a smaller amount of coarse particles, which have a particlediameter of 850 μm or more specified by standard sieve classification,contained is better. The amount is preferably 0 wt % to 5 wt %, morepreferably 0 wt % to 3 wt %, and still more preferably 0 wt % to 1 wt %with respect to the entire water-absorbing resin powder from theviewpoint of the water absorption speed and the like.

Further, the proportion of particles having a particle diameter of 150μm or more and less than 850 μm is preferably 90 wt % or more, morepreferably 95 wt % or more, still more preferably 98 wt % or more, andparticularly preferably 99 wt % or more (the upper limit is 100 wt %)with respect to the entire water-absorbing resin powder from theviewpoint of GCA, FGBP, the water absorption speed, the fluid retentioncapacity under pressure, and the like.

Further, the logarithmic standard deviation (σζ) of the particle sizedistribution is preferably 0.20 to 0.50, more preferably 0.25 to 0.45,and still more preferably 0.30 to 0.40.

When the amount of fine particles which have a particle diameter of lessthan 150 μm contained is adjusted to a small value as the above range,dusting is reduced, handling becomes easier, and GCA, FGBP, and thefluid retention capacity under pressure are improved.

When this range is compared to the particle diameter range adjusted inthe step for pulverizing and classifying and the particle diameter rangeof the water-absorbing agent, the particle diameter at the stage ofgel-crushing is smaller, and thus the dried water-absorbing resinparticles are in a state where the granulated form is highly developed.

That is, when a value obtained by dividing the weight average particlediameter of the water-absorbing agent by the weight average particlediameter of the hydrogel particles after gel-crushing converted to thedried product is increased, the proportion of the granulated particlesshown in FIG. 2 , {granulated particle/(particles havingirregularly-crushed shape +granulated particle)}, is increased. Theweight average particle diameter of the hydrogel particles converted tothe dried product become smaller than the weight average particlediameter of 300 μm to 500 μm set in the particulate absorbing agent(water-absorbing resin powder), and the proportion of the granulatedparticles in the particulate water-absorbing agent is increased as theproportion of small particles is increased (refer to FIG. 2 ).

As compared to FIG. 4 (the water-absorbing resin powder of ComparativeExample 1), as seen in FIG. 2 (the water-absorbing resin powder ofExample 9), it is obvious that the surface area is increased as thegranulated particle ratio of the water-absorbing resin powder isincreased in accordance with a decrease in the weight average particlediameter of the hydrogel particles converted to the dried product.

Further, as compared to FIG. 5 (the water-absorbing resin powder ofComparative Example 5), as seen in FIG. 3 (the water-absorbing resinpowder of Example 9), it is obvious that with use of the adhesioncontrolling agent, the surface area of the granulated particles of thewater-absorbing resin powder is increased.

A BET specific surface area described below in Examples can be employedas an index of the surface area.

The BET specific surface area of the water-absorbing resin powder havingthe same particle size (water-absorbing resin powder passing through asieve having a mesh size of 425 μm and not passing through a sievehaving a mesh size of 300 μm) is preferably 29 m²/kg or more, morepreferably 30 m²/kg or more, and still more preferably 31 m²/kg or more.Incidentally, the water-absorbing resin powder fractionated into aparticle size of 300 μm or more and less than 425 μm is described aswater-absorbing resin powder (425/300) in Table 5.

As described above, control of the particle size can be performed duringpolymerization, during gel-crushing, or during pulverizing orclassification after drying, and is particularly preferably performedduring pulverizing and/or classification after drying. Further, theparticle size is measured using a JIS standard sieve in conformity withthe method specified by WO 2004/69915 A or EDANA-ERT420.2-02.

In order to further solve the problem of the present invention, theparticle size may also be applied to a water-absorbing resin particlesafter surface crosslinking or a particulate water-absorbing agent as afinal product.

Fine particles (for example, particles passing through a wire mesh of150 μm) generated by the control of the particle size may be discardedor recovered according to a recovery method into an aqueous monomersolution before polymerization (WO 92/001008 A and WO 92/020723 A) or arecovery method into a hydrogel during polymerization (WO 2007/074167 A,WO 2009/109563 A, WO 2009/153196 A, and WO 2010/006937 A) as knownconventionally.

Further, the shape of the water-absorbing resin powder of the presentinvention is not limited to a spherical shape, a fibrous shape, a rodshape, a nearly spherical shape, a flat shape, an irregular shape, agranulated particle shape, and a particle having a porous structure, andthe like.

(CRC before Surface Crosslinking)

Since it is preferable that the water-absorbing agent of the presentinvention satisfy CRC≥28 g/g, CRC of the water-absorbing resin powderbefore surface crosslinking is also preferably 28 g/g or more and morepreferably 30 g/g or more. It is sufficient that the amount of thecrosslinking agent during polymerization is appropriately adjusted inthe ranges. CRC of the water-absorbing resin powder before surfacecrosslinking is appropriately adjusted in a range of preferably 30 g/gto 60 g/g, more preferably 32 g/g to 55 g/g, and still more preferably33 g/g to 50 g/g by the amount of the crosslinking agent, polymerizationtemperature, drying temperature, and the like.

In general, polymerization or drying at a high temperature tends toimprove CRC. Further, an increase in the amount of the crosslinkingagent leads to decrease of CRC. Thus, CRC of the water-absorbing agentof the present invention may be appropriately controlled by CRC beforesurface crosslinking, and the amount of the crosslinking agent insurface crosslinking, reaction temperature, and reaction time describedbelow.

(2-6) Surface Crosslinking Step (Step (vi))

This step more specifically includes a step for adding a surfacecrosslinking agent described in (2-6-1) and a heat treatment stepdescribed in (2-6-2) described below.

(2-6-1) Step for Adding Surface Crosslinking Agent

This is a step for mixing the water-absorbing resin powder with asurface crosslinking agent to prepare water-absorbing resin powdercontaining a surface crosslinking agent provided to the surfacecrosslinking step.

In general, surface crosslinking is performed by addition of an organicsurface crosslinking agent described below, polymerization of a monomer(polymerizable surface crosslinking agent) on the surface of thewater-absorbing resin powder, or addition of a radical polymerizationinitiator (surface crosslinking agent in a broad sense) such as apersulfate and heating or irradiation with an ultraviolet ray, or thelike. In the present invention, it is preferable to add an organicsurface crosslinking agent to the water-absorbing resin powder obtainedabove.

(Organic Surface Crosslinking Agent)

As the organic surface crosslinking agent which can be used in thepresent invention, from the viewpoint of the physical properties of theresulting water-absorbing resin particles, an organic compound having areactive group such as a hydroxy group and/or an amino group or the liketo perform a dehydration esterification reaction or a dehydrationamidation reaction with a carboxyl group as a functional group of thepoly(meth)acrylic acid (salt)-based water-absorbing resin particles ispreferable.

The organic compound is not limited to an alcohol compound and an aminecompound directly having a hydroxy group or an amino group, and includeseven a cyclic compound having a reactive group to generate a hydroxygroup or an amino group and/or a reactive group to directly react withthe carboxyl group, such as an alkylene carbonate compound or anoxazolidinone compound.

Examples of the organic surface crosslinking agent include a polyhydricalcohol compound, an epoxy compound, a polyvalent amine compound or acondensate thereof with a haloepoxy compound, an oxazoline compound, a(mono-, di-, or poly-) oxazolidinone compound, an oxetane compound, andan alkylene carbonate compound. A polyhydric alcohol compound, analkylene carbonate compound, an oxazolidinone compound are morepreferable.

Specific examples of the organic surface crosslinking agent include apolyalcohol compound (polyhydric alcohol) such as (di-, tri-, tetra-, orpoly-) ethylene glycol, (di- or poly-) propylene glycol,1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, (poly)glycerin,2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol,1,6-hexanediol, trimethylolpropane, di- or tri-ethanolamine,pentaerythritol, or sorbitol;

an epoxy compound such as (poly)ethylene glycol diglycidyl ether, (di-or poly-) glycerol polyglycidyl ether, or glycidol;

an oxazoline compound such as 2-oxazolidone,N-hydroxyethyl-2-oxazolidone, or 1,2-ethylene bisoxazoline;

an alkylene carbonate compound such as 1,3-dioxolan-2-one (that is,ethylene carbonate), 4-methyl-1,3-dioxolan-2-one,4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one,4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one,1,3-dioxane-2-one, 4-methyl-1,3-dioxane-2-one,4,6-dimethyl-1,3-dioxane-2-one, or 1,3-dioxopan-2-one;

a haloepoxy compound such as epichlorohydrin, epibromohydrin, ora-methylepichlorohydrin, and a polyvalent amine adduct thereof (forexample, Kaimen manufactured by Hercules Inc.; registered trademark);

a silane coupling agent such as γ-glycidoxypropyltrimethoxysilane orγ-aminopropyltriethoxysilane;

an oxetane compound such as 3-methyl-3-oxetane methanol,3-ethyl-3-oxetane methanol, 3-butyl 3-oxetane methanol,3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, 3-butyl 3-oxetaneethanol, 3-chloromethyl-3-methyloxetane, 3-chloromethyl ethyloxetane, ora polyvalent oxetane compound; and a cyclic urea compound such as2-imidazolidinone.

As the polyhydric alcohol, a polyhydric alcohol having 2 to 8 carbonatoms is preferable, a polyhydric alcohol having 3 to 6 carbon atoms ismore preferable, and a polyhydric alcohol having 3 or 4 carbon atoms isstill more preferable. Further, a diol is preferable, and examplesthereof include ethylene glycol, propylene glycol, 1,3-propanediol, and1,4-butanediol. A polyhydric alcohol selected from propylene glycol,1,3-propanediol, and 1,4-butanediol is preferable.

Further, as the epoxy compound, a polyglycidyl compound is preferable,and ethylene glycol diglycidyl ether is suitably used.

As the oxazoline compound, 2-oxazolidinone is suitably used.

As the alkylene carbonate compound, 1,3-dioxolan-2-one (that is,ethylene carbonate) is suitably used.

Furthermore, two or more compounds selected from the polyhydric alcoholcompound, the epoxy compound, the oxazoline compound, and the alkylenecarbonate compound are preferably used in combination. From theviewpoint of higher physical properties, a combination of a polyhydricalcohol and the organic surface crosslinking agent other than thepolyhydric alcohol is preferable, a combination of a polyhydric alcoholand an epoxy compound or an alkylene carbonate compound is morepreferable, and a combination of a polyhydric alcohol and an alkylenecarbonate compound is still more preferable.

In a case where a plurality of the organic surface crosslinking agentsare combined, particularly in a combination of the polyhydric alcoholand the organic surface crosslinking agent other than the polyhydricalcohol, the ratio (the weight ratio) is, which is expressed aspolyhydric alcohol:any compound other than polyhydric alcohol,preferably 1:100 to 100:1, more preferably 1:50 to 50:1, and still morepreferably 1:30 to 30:1.

(Solvent and Concentration)

The total amount of the organic surface crosslinking agent added ispreferably 0.001 parts by weight to 15 parts by weight and morepreferably 0.01 parts by weight to 5 parts by weight with respect to 100parts by weight of the water-absorbing resin powder before addition.

Further, in a case where two kinds of the compounds, that is, apolyhydric alcohol compound and a compound other than the polyhydricalcohol are used as the organic surface crosslinking agent,

the total amount of the polyhydric alcohol compound is preferably 0.001parts by weight to 10 parts by weight and more preferably 0.01 parts byweight to 5 parts by weight with respect to 100 parts by weight of thewater-absorbing resin powder before addition.

Further, the total amount of the compound other than the polyhydricalcohol is preferably 0.001 parts by weight to 10 parts by weight andmore preferably 0.01 parts by weight to 5 parts by weight with respectto 100 parts by weight of the water-absorbing resin powder.

The organic surface crosslinking agent is preferably added as an aqueoussolution. The amount of water used for the aqueous solution ispreferably 0.5 parts by weight to 20 parts by weight and more preferably0.5 part by weight to 10 parts by weight as the total amount withrespect to 100 parts by weight of the water-absorbing resin powderbefore the addition treatment. Incidentally, the amount of water alsoincludes crystal water, hydrated water, and the like of the surfacecrosslinking agent.

Furthermore, a hydrophilic organic solvent may be added to the aqueousorganic surface crosslinking agent solution. In this case, the amount ofthe hydrophilic organic solvent is preferably more than 0 parts byweight and 10 parts by weight or less, and more preferably more than 0parts by weight and 5 parts by weight or less with respect to 100 partsby weight of the water-absorbing resin powder before the additiontreatment. Examples of the hydrophilic organic solvent include a primaryalcohol having 1 to 4 carbon atoms, especially a primary alcohol having2 or 3 carbon atoms, and a lower ketone having 4 or less carbon atomssuch as acetone. Particularly, a volatile alcohol having a boiling pointof lower than 150° C., more preferably lower than 100° C., is preferablebecause the volatile alcohol evaporates during a surface crosslinkingtreatment and no residue is left.

Specific examples thereof include lower alcohols such as methyl alcohol,ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol,isobutyl alcohol, and t-butyl alcohol; ketones such as acetone; etherssuch as dioxane, tetrahydrofuran, and methoxy (poly)ethylene glycol;amides such as e-caprolactam and N,N-dimethylformamide; sulfoxides suchas dimethyl sulfoxide; and polyhydric alcohols such as polyoxypropyleneand oxyethylene-oxypropylene block copolymers.

Further, upon mixing a surface crosslinking agent solution to thewater-absorbing resin powder, water-insoluble fine particles or asurfactant may be present together within a range not impairing theeffect of the present invention, in an amount of more than 0 parts byweight and 10 parts by weight or less, preferably more than 0 parts byweight and 5 parts by weight or less, and more preferably more than 0parts by weight and 1 part by weight or less with respect to 100 partsby weight of the water-absorbing resin powder before the additiontreatment. In this case, a surfactant used or the like is disclosed inU.S. Pat. No. 7,473,739 or the like.

The concentration of the surface crosslinking agent in the aqueoussurface crosslinking agent solution is determined appropriately, and is1 wt % to 80 wt %, 5 wt % to 60 wt %, 10 wt % to 40 wt %, or 15 wt % to30 wt % from the viewpoint of physical properties. As the remnant, thehydrophilic organic solvent or other components are contained.

The temperature of the aqueous organic surface crosslinking agentsolution is determined appropriately from the solubility of the organicsurface crosslinking agent used, the viscosity of the aqueous solution,and the like, and is preferably in a range of −10° C. to 100° C., morepreferably in a range of 5° C. to 70° C., still more preferably in arange of 10° C. to 65° C., and particularly preferably in a range of 25°C. to 50° C.

A high temperature is not preferable because a cyclic compound may behydrolyzed (for example, decomposition from ethylene carbonate toethylene glycol or decomposition from oxazolidinone to ethanol amine),water or a hydrophilic organic solvent may evaporate to reduce amiscibility, or the like, before mixing or reaction with thewater-absorbing resin powder. Further, a too low temperature is notpreferable because the surface crosslinking agent solution may besolidified or the surface crosslinking agent may be precipitated.

(Combined Use of Acid or Base in Surface Crosslinking Agent Solution)

In order to accelerate a reaction or uniform mixing of a surfacecrosslinking agent, the surface crosslinking agent solution may containan acid or a base in addition to the organic surface crosslinking agent,the hydrophilic organic solvent, the surfactant, and the water-insolublefine particles.

As the acid or the base, an organic acid or a salt thereof, an inorganicacid or a salt thereof, and an inorganic base are used, and are usedappropriately in an amount of 0 parts by weight to 10 parts by weight,preferably 0.001 parts by weight to 5 parts by weight, and morepreferably 0.01 parts by weight to 3 parts by weight with respect to 100parts by weight of the water-absorbing resin powder before the additiontreatment. Examples of the organic acid include a water-soluble organicacid having 1 to 6 carbon atoms, preferably 2 to 4 carbon atoms, and awater-soluble saturated organic acid, particularly a saturated organicacid having a hydroxyl group.

Other examples thereof include a non-crosslinkable water-solubleinorganic base (preferably, an alkali metal salt, an ammonium salt, analkali metal hydroxide, and ammonia or a hydroxide thereof) and anon-reducing alkali metal salt pH buffer (preferably a hydrogencarbonate, a dihydrogen phosphate, a hydrogen phosphate, and the like).

(Method for Adding Organic Surface Crosslinking Agent Solution)

The organic surface crosslinking agent is added to the water-absorbingresin powder by an addition treatment. A method for the additiontreatment is not particularly limited. Examples thereof include a methodfor immersing water-absorbing resin powder in a hydrophilic organicsolvent such that the added crosslinking agent is adsorbed thereby, anda method for spraying or dropwise adding a crosslinking agent solutiondirectly to water-absorbing resin powder and mixing. The latter methodis preferable from the viewpoint of adding a predetermined amountuniformly. Furthermore, for the uniform addition, the addition treatmentis preferably performed while water-absorbing resin powder is stirred,and further the organic surface crosslinking agent solution ispreferably sprayed.

In the addition treatment, two or more kinds of the added crosslinkingagents having different compositions may be added simultaneously, forexample, by using different spraying nozzles. However, a singlecomposition is more preferable from the viewpoint of uniformity and thelike. Further, in the case of a single composition, a plurality ofspraying nozzles may be used, considering the size of the additiontreatment apparatus, the treatment amount thereof, the spraying angle ofthe spraying nozzle, or the like.

As an apparatus for use in the addition treatment (hereinafter, referredto as a mixing apparatus in some cases), for example, a cylindricalmixer, a double-wall conical mixer, a V-shaped mixer, a ribbon typemixer, a screw type mixer, a fluidization type furnace, a rotary discmixer, an air current type mixer, a double-arm kneader, an internalmixer, a pulverizing type kneader, a rotary mixer, a screw extruder,Turbulizer, a ploughshare mixer, or the like are suitably used. Further,in large-scale production such as commercial production, the mixingapparatus is preferably an apparatus capable of performing continuousmixing. Furthermore, one and the same apparatus may be used in each ofthe addition treatments, or separate apparatuses may be used in theaddition treatments.

The water-absorbing resin powder to be provided in the present step ispreferably heated or warmed. The temperature of the water-absorbingresin powder is preferably in a range of 10° C. to 100° C., morepreferably in a range of 15° C. to 80° C., and still more preferably ina range of 20° C. to 70° C.

This temperature of 10° C. or higher is preferable because precipitationof the surface crosslinking agent, moisture absorption of thewater-absorbing resin powder or the like is suppressed and the surfacetreatment is performed sufficiently and uniformly. Further, thistemperature of 100° C. or lower is preferable because evaporation ofwater from the aqueous surface crosslinking agent solution is suppressedand a risk such as precipitation of the surface crosslinking agent isreduced.

(2-6-2) Heat Treatment Step

This is a step for performing a heat treatment to perform a crosslinkingtreatment on the surface or the vicinity of the surface of thewater-absorbing resin powder in order to improve fluid retentioncapacity under pressure or GCA of the water-absorbing resin particles.In this regard, an excessive surface crosslinking treatment may lowerCRC too much. Therefore, this step is preferable to perform the surfacecrosslinking treatment until CRC is 28 g/g or more.

The preferable degree of the surface crosslinking can be confirmed by adecrease width of CRC before and after surface crosslinking, thereforethe amount of the surface crosslinking agent and the reactiontemperature/time may be appropriately selected such that a decrease ofCRC caused by surface crosslinking is 0.5 g/g or more, and further 1 g/gto 20 g/g and 2 g/g to 15 g/g.

The heat treatment step may be performed simultaneously with or afterthe step for adding a surface crosslinking agent, and is preferablyperformed after the step for adding a surface crosslinking agent.Further, this step may be performed once, or may be performed multipletimes under the same or different conditions.

The water-absorbing resin particles dried obtained until the step (2-5)has a granulated form. However, before surface crosslinking, thewater-absorbing resin particles are in the form of the granulatedparticles which include primary particles adhered physically each other,and in some cases, the granulated form may be collapsed during swellingto be fragmented and water absorption performance and liquidpermeability ability may be degraded.

However, by carrying out this surface crosslinking step, the crosslinkdensity in the vicinity of the surface of the granulated particle isincreased and further, primary particles (gel particles obtained in thegel-crushing step) that form the granulated particles are chemicallybound so that crosslinking between particles is also achieved while theadhesion of the primary particles is loose or in point contact with eachother. Thus, the particles after surface crosslinking are difficult tocollapse during swelling and thus the object of the present invention isachieved.

In this way, in the present invention, not only addition of the adhesioncontrolling agent but also combination with the surface crosslinkingstep have significance.

(Heating Apparatus)

A heating apparatus as used in the present invention is exemplified by acontinuous or batch type heating apparatus including a known drier or aknown heating furnace and a gas discharge structure and/or a gas supplystructure for obtaining a predetermined atmosphere a predeterminedatmosphere, and is suitably a continuous type heating apparatus. As theheating method of the heating apparatus, a conductive heat transfertype, a radiative heat transfer type, a hot-air heat transfer type, or adielectric heating type are advantageous. The conductive heat transfertype and/or a hot-air heat transfer type heating method are more, andthe conductive heat transfer type heating method is still morepreferable.

The so-called controlled temperature of the heating apparatus may be anytemperature at which the water-absorbing resin powder can be heated to atemperature to be described later, and it is not necessary for thecontrol temperature to be constant from the beginning to the end of theprocess. However, in order to prevent partial overheating or the like,the control temperature of the heating apparatus is preferably 50° C. to300° C. In a case where damage resistance is regarded as important amongthe physical properties of the resulting water-absorbing resin particlesand water-absorbing agent, the controlled temperature is more preferably250° C. or lower, still more preferably 70° C. to 230° C., and stillmore preferably 90° C. to 220° C.

On the other hand, in a case where the water absorption performance isregarded as important, the controlled temperature is more preferably120° C. to 280° C., still more preferably 150° C. to 250° C., andparticularly preferably 170° C. to 230° C.

Further, the heating time is preferably 1 minute to 180 minutes, morepreferably 5 minutes to 120 minutes, still more preferably 10 minutes to120 minutes, and still more preferably 15 minutes to 60 minutes. Whenthe time for the heat treatment is shorter than 1 minute, the surfacecrosslinking treatment is not sufficient so that the fluid retentioncapacity under pressure (AAP) is decreased. On the other hand, when thetime for the heat treatment is too long, coloration may occur or thefluid retention capacity without pressure (CRC) may be excessivelydecreased.

Further, in order to enhance the efficiency of heating and perform auniform heat treatment, it is preferable to use an apparatus including astructure for continuously stirring and/or fluidizing the heatingobject. A stirring and/or a fluidizing method is preferably a groovestirring method, a method of a screw type, a method of a rotary type, amethod of a disc type, a method of a kneading type, a method of afluidized tank type, and the like, and more preferably a stirring methodusing stirring blades (paddles) and a stirring method based on movementof a heat transfer surface itself such as a rotary retort furnace.Incidentally, the stirring and/or fluidizing structure aims perform auniform heat treatment, and therefore need not to be used in a casewhere the treated amount is small, for example, in a case where thedrying object has a thickness of less than 1 cm.

The discharge structure corresponds to an air exit. If gas is dischargedfrom an outlet of a heated product, the outlet also corresponds to thedischarge mechanism. Further, it is preferable that the dischargestructure adjust the amount of gas discharged therefrom and a pressureof the gas discharged therefrom by means of a blower or the like.Furthermore, an air exit point is not limited to one air exit point. Aplurality of air exit points can be provided in consideration of thesize of the heating apparatus and an adjustment status of the dew pointand temperature. The heating apparatus includes the gas supply structureand can also control an atmospheric dew point and an atmospherictemperature in the heating portion by adjusting the structure, forexample, by adjusting the amount of gas supplied.

It is preferable that the gas pressure of the heating portion beslightly lower than a normal pressure. Such a differential pressure ispreferably in a range of 0 kPa to −10 kPa, more preferably in a range of0 kPa to −5 kPa, and still more preferably in a range of 0 kPa to −2 kPawith respect to atmospheric pressure. In making industrial continuousproduction, it is possible to use a batch processing-type or continuousprocessing-type heating apparatus including the above structure.

Incidentally, when the addition treatment of an additive is performedbefore or after the heat treatment or before and after the heattreatment, one and the same apparatus as used in the above additiontreatment may be used in these addition treatments. Alternatively,separate apparatuses may be used in these addition treatments.Particularly, in a case where a production apparatus of continuous typeis used, it is preferable in terms of production efficiency that one andthe same apparatus be used both in the addition treatment performedbefore heating and the heat treatment while another apparatus is used inthe addition treatment performed after heating.

Further, for the purpose of preventing the occurrence of excessivecrosslinking reaction and improving handling property in a subsequentstep, the water-absorbing resin particles taken out of the heatingapparatus may be cooled, as required, to a temperature of preferablylower than 100° C., and more preferably 0° C. to 95° C. or 40° C. to 90°C.

(2-7) Addition of Liquid Permeability Enhancer (Step (vii))

This is a step for adding a liquid permeability enhancer for increasingFGBP and is preferably performed during or after the surfacecrosslinking step.

(Liquid Permeability Enhancer)

The liquid permeability enhancer in the present invention is an additiveselected from insoluble fine particle compounds and polyvalent cationiccompounds or an additive for increasing FGBP as compared with a casewhere no liquid permeability enhancer is used.

The water-insoluble fine particle compound and the cationic compound inthe present invention serve as a stereoscopic spacer or an electrostaticspacer on the surface of the water-absorbing resin particles. Thewater-insoluble fine particle compound and the cationic compound causethe resulting water-absorbing agent to have “increased liquidpermeability (for example, FGBP increased by 10×10⁻⁹ cm² or more,preferably 30×10⁻⁹ cm² or more, and more preferably 50×10⁻⁹ cm² or moreas compared with FGBP obtained when these compounds are not used).”

Besides, these additives, depending on their kinds, can achieve effectssuch as “Anti-Caking,” “deodorization/antibacterial activity,” and“reduction of a residual surface crosslinking agent,” but their effectsand intended uses are not particularly limited in the present invention.

The liquid permeability enhancer essentially added in the productionmethod according to the present invention is preferably selected fromwater-insoluble inorganic fine particles and polyvalent cationiccompounds (cationic polymer compounds or water-soluble polyvalent metalcation-containing compounds).

According to the preferred embodiment of the present invention, theliquid permeability enhancer corresponds to the water-insolubleinorganic fine particles. According to such an embodiment, technicaleffectiveness of improvement in liquid permeability is achieved.

The “water soluble” compound used in the present specification refers toa compound that dissolves in an amount of 1 g or more or 5 g or morewith respect to 100 g of water at 25° C. The “water insoluble” compoundrefers to a compound that dissolves in only an amount of less than 1 g,less than 0.5 g or less than 0.1 g with respect to 100 g of water at 25°C.

In the present invention, while the organic surface crosslinking agentcrosslinks with a functional group of the water-absorbing resin powderby covalent bond, the polyvalent cationic compound (cationic polymercompound or water-soluble polyvalent metal cation-containing compound),which is preferably used as the liquid permeability enhancer in thepresent invention, is assumed to crosslink with the water-absorbingresin powder or the water-absorbing resin particles by ion crosslinkingor is assumed to serve as a stereoscopic spacer or an electrostaticspacer, thereby improving liquid permeability.

(Inorganic Fine Particles)

Examples of the inorganic fine particles include: water-insoluble fineparticulate inorganic powder such as silicon dioxide, titanium dioxide,aluminum oxide, magnesium oxide, zinc oxide, talc, metal phosphate (forexample, calcium phosphate, barium phosphate, and aluminum phosphate),metal borate (for example, titanium borate, aluminum borate, ironborate, magnesium borate, manganese borate, and calcium borate), silicicacid or a salt thereof, clay, diatomaceous earth, zeolite, bentonite,kaolin, hydrotalcite, and activated clay; and organic fine powder suchas calcium lactate, aluminum lactate, and a metal soap (polyvalent metalsalt of long chain fatty acid). The volume average particle diameter ofthe inorganic fine particles is preferably 10 μm or less and morepreferably 1 μm or less.

The inorganic fine particles may be mixed in the form of powder with thewater-absorbing resin powder or the water-absorbing resin particles ormay be mixed in the form of a water dispersion (slurry, for example,colloidal silica) with the water-absorbing resin powder or thewater-absorbing resin particles. Alternatively, the inorganic fineparticles may be mixed in the form of being dispersed in the surfacecrosslinking agent or the aqueous solution of the surface crosslinkingagent with the water-absorbing resin particles.

(Cationic Polymer Compound)

The cationic polymer compound is not particularly limited, but cationicpolymer compounds described in U.S. Pat. Nos. 5,382,610, 7,098,284, WO2009/110645 A, WO 2009/041731 A, and WO 2009/041727 A can be suitablyused.

Among the compounds described in the above-listed documents, thecationic polymer compound in the present invention is preferablypolyethylene imine, polyvinyl amine, polyallylamine, or a condensate ofdimethylamine/ammonia/epichlorohydrin.

As for a molecular weight of the cationic polymer compound, a weightaverage molecular weight is preferably 1,000 to 5,000,000, morepreferably 2,000 to 1,000,000, and still more preferably 10,000 to500,000.

The cationic polymer compound is preferably water soluble from theviewpoint of facilitating mixing. Here, the term “water soluble” meansto be able to dissolve in an amount of 1 g or more with respect to 100 gof water at 25° C.

The cationic polymer compound may be directly mixed with thewater-absorbing resin particles or may be mixed in the form of asolution, particularly in the form of an aqueous solution.Alternatively, the cationic polymer compound may be mixed in the form ofbeing dissolved in the surface crosslinking agent or in the aqueoussolution of the surface crosslinking agent.

(Water-Soluble Polyvalent Metal Cation-Containing Compound)

The water-soluble polyvalent metal cation-containing compound refers toa compound containing a bivalent or higher metal cation, preferably atrivalent or higher metal cation. The trivalent or higher metal cationis exemplified by aluminum, zirconium, and titanium. Among these,aluminum is preferable.

Examples of the polyvalent metal cation-containing compound includepolyvalent metal compounds, which are inorganic surface crosslinkingagents, including inorganic salts of polyvalent metals such as aluminumsulfate, aluminum chloride, zirconium chloride oxide, zirconium ammoniumcarbonate, zirconium potassium carbonate, zirconium potassium carbonate,zirconium sulfate, zirconium acetate, and zirconium nitrate; and organicsalts of polyvalent metals such as aluminum acetate, aluminum lactate,hydroxy zirconium chloride, titanium triethanol aminate, and titaniumlactate.

Among theseas the polyvalent metal cation ,a compound containingaluminum is preferable.

These compounds may be directly mixed in the form of powder with thewater-absorbing resin particles, may be mixed in the form of a solutionor a dispersion, particularly in the form of an aqueous solution, or maybe mixed in the form of being dissolved in the surface crosslinkingagent or the aqueous solution of the surface crosslinking agent.

In the production method according to the present invention, thewater-soluble polyvalent metal cation-containing compound may be addedtwice or more times. For example, in a case where the water-solublepolyvalent metal cation-containing compound is added twice, a ratiobetween a first addition and a second addition (a first addition/asecond addition) is defined in a range of 1/99 to 99/1 and preferably ina range of 10/90 to 90/10. A ratio falling outside the above ranges isnot preferable because it is extremely close to one-time addition, whichreduces effectiveness of a plurality of additions.

In the meantime, a non-metallic ion crosslinking agent such as acationic polymer compound may express tackiness at the aforementionedmixing. Therefore, the addition of the non-metallic ion crosslinkingagent is preferably performed after the last heat treatment.

In a case where a solvent is used for mixing of the water-solublepolyvalent metal cation-containing compound, the solvent is preferablywater or the aqueous crosslinking agent solution. For improvement indispersity, solubility, and blendability, water may be used incombination with a hydrophilic organic solvent (alcohol or polyglycol)or a surfactant, if necessary.

The amount of water used is appropriately determined according to a kindof additive and an addition method, for example, the amount of waterused is 0 parts by weight (drying blending) to 50 parts by weight, 0.1parts by weight to 10 parts by weight, or 0.5 parts by weight to 5 partsby weight with respect to 100 parts by weight of the water-absorbingresin particles.

Further, as a liquid permeability enhancer other than the above liquidpermeability enhancers, a water-soluble polysiloxane described in WO2009/093708 A, primary to tertiary amine compounds described in WO2008/108343 A, or the like are preferably used.

The amount of the liquid permeability enhancer is preferably 0.001 partsby weight to 5 parts by weight, more preferably 0.002 parts by weight to2 parts by weight, and still more preferably 0.005 parts by weight to 1part by weight with respect to 100 parts by weight of thewater-absorbing resin particles to be added.

Incidentally, for the water-soluble polyvalent metal cation-containingcompound, these values are expressed in terms of the amount ofpolyvalent metal cation (for example, for the aluminum sulfate, it is avalue based on the amount of Al³⁺).

Further, as for the addition timing, the liquid permeability enhancer isappropriately added after pulverization and before surface crosslinking,during surface crosslinking, or after surface crosslinking.

(2-8) Step for Adding Other Additives

This is a step for adding other additives in order to provide variousfunctions to the water-absorbing resin powder or the surface crosslinkedwater-absorbing resin particles and includes one or two or more steps.Examples of the additives include a deodorant, a perfume, antimicrobialagent, a foaming agent, a chelating agent such as trisodiumdiethylenetriamine pentaacetate, or pentasodium diethylenetriaminepentaacetate, a surfactant, a coloring preventing agent, a pigment, adye, a fertilizer, an oxidizing agent, a reducing agent and the like,these additives can provide or enhance functions. Further, as foraddition of the additives, the additives may be added in a state of asolution or added by dry blending.

This step may be performed between any steps of the steps (i) to (vii),and may be simultaneously performed with any steps of the steps (i) to(vii). Preferably, this step is performed during the step (vi) or afterthe step (vi).

The proportion of these additives used is less than 10 wt %, preferablyless than 5 wt %, and more preferably less than 1 wt % of thewater-absorbing resin powder or the surface crosslinked water-absorbingresin particles. Further, these additives may be simultaneously addedwith the surface crosslinking step or may be separately added from thesurface crosslinking step.

In the production method of the present invention, the resultingpolyacrylic acid (salt)-based water-absorbing agent is adjusted to havea surface tension of 60 mN/m or more and fluid retention capacitywithout pressure of 28 g/g or more.

In order to adjust the fluid retention capacitywithout pressure to 28g/g or more, it is preferable to control the crosslink density by thedrying temperature and time of the hydrogel or the degree of the surfacecrosslinking of the water-absorbing resin powder. Further, it ispreferable to control the surface tension by the type of an additive andthe amount of an additive added.

According to the preferred embodiment of the present invention, byadjusting the fluid retention capacity without pressure (CRC) and thesurface tension of the water-absorbing agent to be in a specific range,it is possible to achieve excellent Gel Capillary Absorption (GCA) andFree Gel Bed Permeability (FGBP) and to efficiently achieve the desiredobject of the present invention.

[3] Physical Properties of Poly(Meth)Acrylic Acid (Salt)-BasedParticulate Water-Absorbing Agent

In the present invention, there is also provided a particulatewater-absorbing agent containing polyacrylic acid (salt)-basedwater-absorbing resin particles as a main component, the particulatewater-absorbing agent satisfying the following (1) to (5).

That is, the particulate water-absorbing agent is a polyacrylic acid(salt)-based particulate water-absorbing agent containing polyacrylicacid (salt)-based water-absorbing resin particles as a main componentand satisfying the following (1) to (5): (1) a fluid retention capacitywithout pressure (CRC) is 28 g/g or more; (2) GCA is 28.0 g/g or more;(3) FGBP satisfies, in a case where GCA is in a range of 28.0 g/g ormore and less than 36.0 g/g, mathematical formula:(FGBP=−10×10⁻⁹×GCA+380×10⁻⁹) cm² or more, and FGBP is, in a case whereGCA is 36.0 g/g or more, 30×10⁻⁹ cm² or more; (4) a weight averageparticle diameter (D50) of the particulate water-absorbing agent is 300μm to 500 μm; and (5) a surface tension is 60 mN/m or more.

Incidentally, the water-absorbing agent of the present invention is notlimited by the production method of the present invention as long as itsatisfies the above (1) to (5).

(3-1) Above Physical Property (1) of Water-Absorbing Agent; FluidRetention Capacity Without Pressure (CRC)

The fluid retention capacity without pressure (CRC) of the particulatewater-absorbing agent of the present invention is controlled to be 28g/g or more, more preferably 29 g/g or more, still more preferably 30g/g or more, particularly preferably 31 g/g or more, and most preferably32 g/g or more by appropriately producing internal crosslinking orsurface crosslinking with the above-described production method.

A higher CRC is preferable and there is no particular limitation on theupper limit thereof. However, from the viewpoint of a balance with otherphysical properties (particularly, liquid permeability), CRC ispreferably 50 g/g or less, more preferably 45 g/g or less, and stillmore preferably 42 g/g or less.

CRC can be controlled by adjusting the type and the amount of acrosslinking agent during polymerization or in surface crosslinking inthe ranges described in the above (2-1) to (2-6).

In order to achieve the range of GCA according to the present invention,it is preferable to control the fluid retention capacity withoutpressure (CRC) in the above-described ranges.

(3-2) Above Physical Property (2) of Water-Absorbing Agent; GelCapillary Absorption (GCA)

GCA evaluates liquid absorption ability under a load of 0.05 psi for 10minutes with a difference in height of 10 cm between the upper surfaceof a glass filter and the meniscus at the lower portion of a Mariottetube. GCA evaluates absorption ability for as short time as 10 minutes.The conventionally known fluid retention capacity under pressure (AAP)or FHA described in U.S. Pat. No. 7,108,916 evaluates absorption abilityin a saturation state for 1 hour, and thus is an evaluation method basedon a different idea from GCA according to the present invention. Ahigher value of GCA of a particulate water-absorbing agent improves theability of absorbing urea from pulp in a disposable diaper, can reduce are-wet amount, and can suppress skin rash or urine leakage.

The value of GCA of the particulate water-absorbing agent of the presentinvention is calculated by the method described in Examples below. Ahigher value thereof indicates better performance, and higher values of28.0 g/g or more, 29.0 g/g or more, and 30.0 g/g or more in this orderare preferable. The value is more preferably 31.0 g/g or more, stillmore preferably 31.5 g/g or more, still more preferably 33/g or more,and most preferably 34 g/g or more. A higher upper limit of GCA is morepreferable. However, the upper limit of GCA is usually preferably about50.0 g/g from the viewpoint of a balance with other physical properties.

Therefore, according to the preferred embodiment of the presentinvention, GCA is 31.0 g/g or more. According to such an embodiment, there-wet amount of liquid can be decreased and the technical effect ofimproving the speed of absorbing liquid is achieved.

Incidentally, in the preset invention, it is particularly important thatGCA is in the above range. In addition, it is preferable that the fluidretention capacity under pressure is high, and the water absorptionspeed is high (the water absorption time by the Vortex method is short).

(3-3) Physical Property (3) of Water-Absorbing Agent; FGBP

FGBP is a method of evaluating physiological saline solution penetratingability of the gel layer after a physiological saline solution is pouredfrom the upper portion of the gel layer in a state where a load of 0.3psi is applied to the water-absorbing agent layer freely swollen in acell having a mesh structure on the bottom surface. When a value of FGBPis higher, the speed of absorbing liquid and the re-wet amount in theabsorbent material having a high concentration of the absorbing agentcan be decreased.

In the present invention, in a case where GCA is in a range of 28.0 g/gor more and less than 35.0 g/g, it is preferable to satisfyFGBP≥−10×10⁻⁹×GCA+380×10⁻⁹ cm² (formura 1).

Further, in the case of GCA≥35.0 g/g, it is preferable to satisfyFGBP≥30×10⁻⁹ cm², more preferable to satisfy FGBP≥50×10⁻⁹ cm², and stillmore preferable to satisfy FGBP≥75×10⁻⁹ cm².

Meanwhile, in consideration of only FGBP, it is more preferable tosatisfy FGBP≥100×10⁻⁹ cm², still more preferable to satisfyFGBP≥120×10⁻⁹ cm², still more preferable to satisfy FGBP≥140×10⁻⁹ cm²,still more preferable to satisfy FGBP≥160×10⁻⁹ cm², particularlypreferable to satisfy FGBP≥200×10⁻⁹ cm², and most preferable to satisfyFGBP≥300×10⁻⁹ cm². A higher upper limit of FGBP is more preferable, butthe upper limit of FGBP is usually preferably about 500×10⁻⁹ cm² fromthe viewpoint of a balance with other physical properties.

Incidentally, in the preset invention, it is particularly important thatFGBP is in the above range. In addition, it is preferable that the fluidretention capacity under pressure is high, and the water absorptionspeed is high (the water absorption time by the Vortex method is short).

(3-4) Physical Property (4) of Water-Absorbing Agent; Particle Size

The weight average particle diameter (D50) of the particulatewater-absorbing agent of the present invention is preferably 300 μm to500 μm, more preferably 310 μm to 480 μm, and still more preferably 320μm to 450 μm.

Therefore, according to the preferred embodiment of the presentinvention, the weight average particle diameter (D50) of the polyacrylicacid (salt)-based particulate water-absorbing agent is 300 μm to 500 μm.According to such an embodiment, GCA, FGBP, and the fluid retentioncapacity under pressure can be improved.

As for other preferred particle size characteristics, the amount of fineparticles, which have a particle diameter of less than 150 μm, containedin the particulate water-absorbing agent is preferably 0 wt % to 5 wt %,more preferably 0 wt % to 3 wt %, and still more preferably 0 wt % to 2wt % in 100 wt %.

Furthermore, the amount of coarse particles, which have a particlediameter of 850 μm or more, contained in the particulate water-absorbingagent is preferably 0 wt % to 5 wt %, more preferably 0 wt % to 3 wt %,and still more preferably 0 wt % to 1 wt % of the whole particles.

Further, the proportion of particles having a particle diameter of 150μm or more and less than 850 μm in the particulate water-absorbing agentis preferably 90 wt % or more, more preferably 95 wt % or more, stillmore preferably 98 wt % or more, and particularly preferably 99 wt % ormore (the upper limit is 100 wt %) of the whole particles.

Further, the logarithmic standard deviation (σζ) of the particle sizedistribution of the particulate water-absorbing agent is preferably 0.20to 0.50, more preferably 0.25 to 0.45, and still more preferably 0.30 to0.40.

(3-5) Physical Property (5) of Water-Absorbing Agent; Surface Tension

The surface tension (specified by the measurement method in Examples) ofthe particulate water-absorbing agent of the present invention is 60mN/m or more, more preferably 61 mN/m or more, still more preferably 62mN/m or more, and may be 63 mN/m or more or 64 mN/m or more. Usually, asthe upper limit, 75 mN/m is enough.

Conventionally, when a large amount of a surfactant or a hydrophobicsubstance (for example, 0.1 to 10 wt %) is used during gel-crushing inorder to control the water absorption speed as in Literature 13, therehave been problems that the surface tension of the obtainedwater-absorbing resin particles is lowered (particularly less than 60mN/m, furthermore less than 55 mN/m) and that the re-wet amount of adisposable diaper is increased.

In the present invention, it is particularly important to control thesurface tension in the above range. Therefore, as the control method,the surface tension can be controlled by adjusting the structure, HLB,and the addition amount of the adhesion controlling agent described inthe above (2-3-2) in the above-described ranges.

Incidentally, the method for controlling the surface tension of theparticulate water-absorbing agent of the present invention to 60 mN/m ormore is not particularly limited, but a method of adjusting the type andthe addition amount of the adhesion controlling agent or the like isexemplified.

(3-6) More Preferred Physical Property (1); Fluid Retention CapacityUnder Pressure (AAP)

As described in Examples below, the fluid retention capacity underpressure of the particulate water-absorbing agent of the presentinvention is specified as a fluid retention capacity with respect to a0.90 wt % aqueous sodium chloride solution under a pressure of 2.06 kPa,and is controlled to preferably 24 g/g or more, more preferably 25 g/gor more, still more preferably 26 g/g or more, particularly preferably27 g/g or more, and most preferably 28 g/g or more.

A higher upper limit of AAP is more preferable, but the upper limit ofAAP is usually preferably about 40 g/g from the viewpoint of a balancewith other physical properties.

If GCA is improved within the range specified in the present inventionand the fluid retention capacity under pressure (AAP) can be controlledin the above range, performance of a disposable diaper can be furtherimproved.

(3-7) More Preferred Physical Property (2); Water Absorption Time(Vortex Method)

The water absorption time (Vortex method) of the particulatewater-absorbing agent of the present invention is preferably 40 secondsor shorter, more preferably 35 seconds or shorter, still more preferably30 seconds or shorter, still more preferably 28 seconds or shorter,still more preferably 26 seconds or shorter, still more preferably 24seconds or shorter, particularly preferably 22 seconds or shorter, andmost preferably 19 seconds or shorter.

When GCA and FGBP are improved within the range specified in the presentinvention and the water absorption time (Vortex method) can becontrolled in the above range, performance of a disposable diaper can befurther improved.

(3-8) More Preferred Physical Property (4); Moisture Content

The moisture content (specified by an weight lost from drying at 180°C.×3 hours) of the particulate water-absorbing agent of the presentinvention is not particularly limited as long as it satisfies theabove-described physical properties, but is adjusted to 0.1% to 20%,further 1% to 15%, and particularly 2% to 10%. When the moisture contentis high, physical properties are difficult to satisfy. When the moisturecontent is low, the water absorption speed tends to be decreased andabrasion resistance of particles tends to deteriorate.

According to the preferred embodiment of the present invention, one ormore compounds selected from a nonionic substance, an amphotericsubstance, an anionic substance, and a cationic substance are containedin the inside and/or the surface of the polyacrylic acid (salt)-basedparticulate water-absorbing agent, the nonionic substance is (a) apolyol, (b) a hydroxy group-modified product of a polyol, (c) side-chainand/or terminal polyether-modified polysiloxane, or (d) an alkyleneoxide adduct of higher aliphatic amine, the amphoteric substance is (e)alkylaminobetaine or (f) alkylamine oxide, the anionic substance is (g)a sulfuric acid ester salt of a higher alcohol alkylene oxide adduct or(h) alkyl diphenyl ether disulfonate, and the cationic substance is (i)an ammonium salt.

According to the preferred embodiment of the present invention, thepolyacrylic acid (salt)-based particulate water-absorbing agent furthercontains the liquid permeability enhancer. According to such anembodiment, the technical effect of improving liquid permeability isachieved.

[4] Absorbent Article

An application of the particulate water-absorbing agent of the presentinvention is not particularly limited. The particulate water-absorbingagent of the present invention is preferably used for an absorbent bodyused for disposable diapers or sanitary napkins.

In the present invention, an absorbent material means an absorbingmaterial formed by using the particulate water-absorbing agent of thepresent invention and a hydrophilic fiber as a main component. In theabsorbent material of the present invention, the amount of theparticulate water-absorbing agent contained (core concentration) ispreferably 20 wt % to 100 wt %, more preferably 30 wt % to 95 wt %, andparticularly preferably 50 wt % to 90 wt % with respect to the totalweight of the particulate water-absorbing agent and the hydrophilicfiber. As described above, in the case of using a water-absorbing agenthaving improved GCA, there are problems that the effect of reducing there-wet amount is exerted in the absorbent material containing awater-absorbing agent in a small amount (less than 20 wt %), however, ina high-concentration absorbent material or an absorbent material withoutuse of pulp, the expected effect is not always recognized in view of thespeed of absorbing liquid and the re-wet amount. On the other hand, inthe present invention, by highly achieving a balance between GCA andFGBP, not only the speed of absorbing liquid can be improved but alsothe re-wet amount of liquid can be reduced even in thehigh-concentration absorbent material or the absorbent material withoutuse of pulp.

Further, in a case where the absorbent material of the present inventionis thin, the thickness of the absorbent material is preferably as thinas 1 mm to 5 mm. A thin absorbent article can be obtained by using sucha thin absorbent material. For example, an absorbent article includingthe above-described thin absorbent material of the present invention, asurface sheet having liquid permeability, and a back sheet having liquidimpermeability is obtained.

A method for producing a thin absorbent article of the present inventionmay be as follows. An absorbent article, particularly a disposablediaper or a sanitary napkin, may be formed, for example, by forming anabsorbent material (absorbent core) by blending or sandwiching a fibersubstrate and a particulate water-absorbing agent, sandwiching theabsorbent material with a substrate such as a surface sheet havingliquid permeability and a substrate such as a back sheet having liquidimpermeability, and providing an elastic member, a diffusion layer, anadhesive tape, or the like, as required. Such an absorbent article iscompressed and shaped so as to have a density of 0.06 g/cc to 0.50 g/ccand a basis weight of 0.01 g/cm² to 0.20 g/cm². Incidentally, examplesof the fiber substrate used may include a hydrophilic fiber such as acrushed wood pulp, a cotton linter, a crosslinked cellulose fiber,rayon, cotton, wool, acetate, and vinylon. An air-laid substrate thereofis preferable.

The particulate water-absorbing agent of the present invention exhibitsexcellent absorption characteristics. Therefore, specific examples ofthe absorbent article of the present invention include a hygienicmaterial such as disposable diapers for adults, which are significantlygrowing recently, diapers for children, and sanitary napkins and aso-called incontinence pads. The leakage amount or skin rash is reduceddue to the particulate water-absorbing agent of the present invention inthe absorbent article. Therefore, burden on a person wearing theabsorbent article and nursing people can be largely reduced.

According to the preferred embodiment of the present invention, thehygienic material contains the above-described polyacrylic acid(salt)-based particulate water-absorbing agent.

[5] Comparison to Above Related Arts

Hereinbefore, although there are many parameter-controlledwater-absorbing resins and the present inventors have filed PatentLiterature 7 which is focused on Gel Capillary Absorption (GCA) as a newparameter as compared with the related prior arts such as PatentLiteratures 1 to 6, insufficient points have still been found. In thisregard, in order to solve the above-described problem, the presentinventors have conducted intensive studies. As a result, liquidpermeability has still been insufficient in Patent Literature 7 which isfocused on Gel Capillary Absorption (GCA) as a new parameter. Thepresent inventors have found that the above-described problem can besolved when Free Gel Bed Permeability (FGBP) as a newer index of liquidpermeability is high in addition to GCA, thereby providing the newwater-absorbing agent described above and the production methodtherefor.

While many parameter-controlled water-absorbing resins have beenproposed, in conventional water-absorbing agents disclosed in PatentLiteratures 1 to 6, proceeding Patent

Literature 7, and the like, there is neither suggestion nor disclosureon the water-absorbing agent of the present invention in which a balancebetween GCA and FGB is achieved. Further, although gel-crushing afterpolymerization or during polymerization in the production process forthe water-absorbing resin has been often proposed in the above PatentLiteratures 10 to 21, and the like, there is no disclosure in therelated prior arts that an adhesion controlling agent is used, theparticle diameter after gel-crushing is adjusted to be significantlysmall, and a liquid permeability enhancer is thereafter used.

The present invention provides a new production method in which theparticle diameter after gel-crushing is adjusted to be significantlysmall and an adhesion controlling agent and a liquid permeabilityenhancer are used. Providing the new water-absorbing agent of thepresent invention by such a new production method described above is asdescribed in the above specification and the following Examples.

EXAMPLES

Hereinafter, the invention is described according to the Examples, butthe present invention should not be construed as being limited to theExamples. Various physical properties described in the claims of thepresent invention or Examples were determined according to the followingmeasurement methods (a) to (i). Incidentally, unless otherwisespecified, each step in each Example was performed substantially at anormal pressure (±5% of the atmospheric pressure, preferably within 1%thereof), and was performed without pressure change by intentionallyincreasing or reducing the pressure in the same step.

(a) Fluid Retention Capacity Without Pressure (CRC) (ERT441.1-02)

The fluid retention capacity without pressure (CRC) was measured inconformity with ERT441.2-02. That is, 0.200 g (weight W0 (g)) of asample was weighed and uniformly placed in an unwoven fabric bag (60mm×85 mm) and the bag was heat-sealed. Then, the bag was immersed in 500mL of a 0.90 wt % aqueous sodium chloride solution, the temperature ofwhich was adjusted to 23° C.±2° C. After 30 minutes, the bag was pulledout and drained by using a centrifugal separator (centrifugemanufactured by KOKUSAN Co., Ltd., type H-122) at 250 G for 3 minutes.Thereafter, a weight (W1 (g)) of the bag was measured. A bag containingno sample was subjected to the similar operation, and a weight (W2 (g))of the bag was measured. The fluid retention capacity without pressure(CRC) was calculated according to the following (formula 1) based on theobtained WO (g), W1 (g), and W2 (g).

[Mathematical Formula 3]

CRC (g/g)={(W1−W2)/W0}−1   (Formula 1)

Gel CRC was obtained through the similar operation to above descriptionexcept that 0.6 g of hydrogel particles or the crosslinked hydrogelpolymer was used as a sample and a free swelling time was set to 24hours. Furthermore, the solids content of the resin of the crosslinkedhydrogel polymer or the hydrogel particles was measured separately so asto obtain a weight of the water-absorbing resin in 0.6 g of thecrosslinked hydrogel polymer or the hydrogel particles. Gel CRC wascalculated based on Formula (2) below. Incidentally, each sample wasmeasured five times, and an average of values obtained by themeasurement was employed.

Gel CRC (g/g)=[{(mwi−mb)/msi}−1]×(100/Wn)   [Mathematical Formula 4]

Incidentally, herein,

msi: a weight (g) of the crosslinked hydrogel polymer or the hydrogelparticles before measurement

mb: a weight (g) of Blank (unwoven fabric only) which has freely swollenand been drained

mwi: a total weight (g) of the crosslinked hydrogel polymer and theunwoven fabric which have freely swollen and been drained

Wn: solids content (wt %) of the crosslinked hydrogel polymer or thehydrogel particles

(b) Fluid Retention Capacity Under Pressure (AAP) (ERT442.2-02)

The fluid retention capacity under pressure (AAP) of the particulatewater-absorbing agent according to the present invention was measured inconformity with ERT442.2-02. That is, 0.900 g (weight W3 (g)) of theparticulate water-absorbing agent was put into a measurement apparatus,and the weight (W4 (g)) of the whole measurement apparatus was measured.Next, 0.90 wt % of aqueous sodium chloride solution, the temperature ofwhich was adjusted to 23° C.±2° C., was absorbed under a load of 2.06kPa (0.3 psi, 21 g/cm²). After 1 hour, the weight (W5 (g)) of the wholemeasurement apparatus was measured, and the fluid retention capacityunder pressure (AAP) was calculated according to the following (formula3) based on the obtained W3 (g), W4 (g), and W5 (g).

[Mathematical Formula 5]

AAP (g/g)=(W5−W4)/W3   (formula 3)

(c) Water Absorption Time (Vortex Method)

0.02 parts by weight of Food Blue No. 1 that is a food additive wasadded to 1,000 parts by weight of a 0.90 wt % aqueous sodium chloridesolution prepared in advance, and the liquid temperature was adjusted to30° C. 50 ml of the 0.90 wt % aqueous sodium chloride solution coloredin blue was measured and put into a 100 ml beaker. 2.00 g of theparticulate water-absorbing agent was put thereinto while the aqueoussodium chloride solution was stirred at 600 rpm with a cylindricalstirrer having a length of 40 mm and a thickness of 8 mm. The waterabsorption time (second) was measured. The end point was in conformitywith a standard described in JIS K 7224-1996 fiscal year “Testing methodfor water absorbent speed of water-absorbing resins.” A period of timeuntil the water-absorbing agent absorbs the physiological salinesolution and the test liquid covers a stirrer chip was measured as thewater absorption time (second).

(d) Particle Size Distribution, Weight Average Particle Diameter (D50),and Logarithmic Standard Deviation (σζ) of Water-Absorbing Agent(Water-Absorbing Resin Powder)

The particle size (PSD) and the logarithmic standard deviation (σζ) inthe particle size distribution of the particulate water-absorbing agent(water-absorbing resin powder) according to the present invention weremeasured in conformity with the measurement method disclosed in US2006/204755 A.

That is, 10.00 g of the sample was classified using a JIS standard sieve(The IIDA TESTING SIEVE: internal diameter of 80 mm; JIS Z8801-1 (2000))having a mesh size of 850 μm, 600 μm, 500 μm, 425 μm, 300 μm, 150 μm, or45 μm or a sieve corresponding to the JIS standard sieve. After theclassification, the weight of each sieve was measured, and the weightpercent (wt %) of particles having a particle diameter of less than 150μm was calculated. Incidentally, the term “weight percent of particleshaving a particle diameter of less than 150 μm” refers to the weightratio (%) of particles passing through a JIS standard sieve having amesh size of 150 μm with respect to the whole sample.

Further, regarding the weight average particle diameter (D50), theresidual percentage R of each particle size was plotted on a logarithmicprobability paper and the particle diameter corresponding to R=50 wt %was read as the weight average particle diameter (D50) from this graph.The weight average particle diameter (D50) means a particle diametercorresponding to 50 wt % of the whole particulate water-absorbing agent(sample). Further, the logarithmic standard deviation (σζ) in theparticle size distribution is represented by the following (Formula 4).A smaller σζ value means a narrower particle size distribution.

[Mathematical Formula 6]

σζ=0.5×ln(X2/X1)   (Formula 4)

(X1 represents the particle diameter at R=84.1%, X2 represents theparticle diameter at R=15.9%, and In represents logarithm natural.)

(e) Surface Tension

50 ml of a 0.90 wt % aqueous sodium chloride solution, a temperature ofwhich was adjusted to 20° C., was put into a 100 ml beaker sufficientlywashed. First, the surface tension of the 0.90 wt % aqueous sodiumchloride solution was measured using a surface tension meter (K11automatic surface tension meter manufactured by KRUSS GmbH). In thismeasurement, the surface tension has to be in a range of 71 mN/m to 75mN/m. Subsequently, a sufficiently washed cylindrical stirrer having alength of 25 mm and 0.500 g of the particulate water-absorbing agentwere put into a beaker containing 50 ml of the 0.90 wt % aqueous sodiumchloride solution after the measurement of the surface tension, thetemperature of which was adjusted to 20° C. The resulting mixture wasstirred at 350 rpm for 3 minutes. After 3 minutes, stirring was stopped.After the water-containing particulate water-absorbing agent, which hasbeen left to stand still for 2 minutes, was precipitated, a similaroperation was performed again, and the surface tension of thesupernatant was measured. Herein, when the supernatant does not remainin required volume for measurement after the particulate water-absorbingagent is precipitated because the water absorption speed thereof is highor the absorption capacity thereof is high, measurement was conducted byappropriately adjusting the amount of 50 ml of the 0.90 wt % aqueoussodium chloride solution to be in the minimum range necessary formeasurement. Incidentally, the present invention employs a plate methodusing a platinum plate. The plate was sufficiently washed with deionizedwater and was heated and washed with a gas burner before eachmeasurement, and used.

(f) Solids Content and Moisture Content

About 1 g (weight W9 (g)) of a water-absorbing resin (water-absorbingagent) (particulate hydrogel) was measured and put into an aluminum cup(weight W8 (g)) having a diameter of the bottom surface of about 5 cm,and was left for 3 hours in a dryer without wind at 180° C. and thendried. The total weight (W10 (g)) of the aluminum cup and thewater-absorbing resin (water-absorbing agent) after drying was measured,and the solids content was determined by the following (Formula 5).Further, the moisture content was determined by the following (Formula6).

[Mathematical Formula 7]

solids content (wt %)={(W10−W8)/W9}×100    (Formula 5)

[Mathematical Formula 8]

moisture content (wt %)=100−solids content (wt %)    (Formula 6)

(g) GCA (Gel Capillary Absorption)

An apparatus and a method for measuring GCA is described with referenceto FIG. 1 . A glass filter 2 used in this measurement method is a 500 mlglass filtration apparatus as specified by ISO 4793 (1980), has a porediameter of P40 (16 μm to 40 μm) and a thickness of 7 mm, and is, forexample, a Duran grade 3 glass filtration apparatus manufactured bySchott Inc. Further, the filter having a radius of 30 cm has to have awater flowing ability of 50 ml/min at 20° C. at a pressure difference of50 mbar. A silicone tube 3 is connected to the lower part of thefiltration apparatus 1 with the glass filter, and is further connectedto the lower part of a tank 6 provided with a glass tube 5 and a stopcock 4. At this time, the upper surface of the glass filter is fixed ata position 10 cm higher than the meniscus of the lower part of the glasstube in the tank. The system is filled with 0.90 wt % of aqueous sodiumchloride solution. A high humidity strength cellulose tissue 8 cut intoan 8 cm square is fixed to the bottom of a plastic support cylinder 7having an inner diameter of 60 mm with a metal ring. The tissue has amaximum basis weight of 24.6 g/m² and a minimum humidity tensilestrength of 0.32 N/cm (CD direction) and 0.8 N/cm (MD direction) (theflowing direction when paper is produced by a paper machine is referredto as the MD direction, and a direction perpendicular thereto isreferred to as the CD direction), and is available from Fripa Inc. inGermany, for example. 100.2 g (weight W11 (g)) of the particulatewater-absorbing agent was scattered uniformly on the tissue under theconditions of a room temperature (20° C. to 25° C.) and a humidity of 50RH %. A piston 9 adjusted so as to uniformly apply a load of 0.39 kPa(0.05 psi) to the water-absorbing agent, having an outer diameter ofslightly less than 60 mm, not causing a gap with the support cylinder,and capable of moving vertically without being hindered, was put on theparticulate water-absorbing agent. The weight of the whole measurementapparatus (W12 [g]) was measured. The whole measurement apparatus wasput on a glass filter. A valve of the fluid tank with a Mariotte tubewas opened for absorption for 10 minutes. Thereafter, the wholemeasurement apparatus was pulled up, and the weight thereof (W13 (g))was measured. GCA (g/g) was calculated according to the following(Formula 7) from W11, W12, and W13.

[Mathematical Formula 9]

GCA (g/g)=(W13−W12)/W11   (Formula 7)

(h) FGBP

FGBP of the present invention was carried out in conformity with a gelbed permeability test under the “free swelling” condition described inWO 2004/096304 A, except that the water-absorbing agent in a range of300 μm to 600 μm was not selected, the particle diameter of thewater-absorbing agent was measured as it was, and a period of time forcollecting data was changed from every 1 second for 20 seconds to every5 seconds for 180 seconds.

(i) Particle Size Distribution and Weight Average Particle Diameter(D50) of Crosslinked Hydrogel Polymer (Hydrogel Particles)

20 g of the crosslinked hydrogel polymer (hydrogel particles) (solidscontent: α wt %) at a temperature of 20° C. to 25° C. was added to 1,000g of a 20 wt % aqueous sodium chloride solution containing 0.08 wt % ofEMAL 20C (surfactant, manufactured by Kao Corporation) (hereinafter,referred to as the “EMAL aqueous solution”) to obtain a dispersion andthe dispersion was stirred with a stirrer chip (length 50 mm X diameter7 mm) at 300 rpm for 16 hours (using a columnar polypropylene vessel,height: 21 cm, diameter: 8 cm, having a capacity of about 1.14 L).

After finishing the stirring, the dispersion was supplied to a centralportion of JIS standard sieves (diameter: 21 cm, mesh sizes of thesieves; 8 mm/4 mm/2 mm/1 mm/0.60 mm/0.30 mm/0.15 mm/0.075 mm) disposedon a turntable. All the crosslinked hydrogel polymer (hydrogelparticles) was washed with use of 100 g of the EMAL aqueous solution sothat the crosslinked hydrogel polymer (hydrogel particles) would appearon the sieves, and then 6,000 g of the EMAL aqueous solution wasuniformly poured from above at the height of 30 cm with use of a shower(with 72 holes, flow rate; 6.0 L/min) while the sieves were manuallyrotated (at 20 rpm) so that the water-pouring range (50 cm²) would coverthe entire sieves. This operation was repeated four times to classifythe crosslinked hydrogel polymer (hydrogel particles). The classifiedcrosslinked hydrogel polymer (hydrogel particles) on a first-stage sievewas drained for about 2 minutes and then weighed. The classifiedcrosslinked hydrogel polymer (hydrogel particles) on second andsubsequent sieves were classified by the similar operation and drained,and the crosslinked hydrogel polymer (hydrogel particles) remaining oneach sieve was weighed. Incidentally, in a case where the hydrogelparticle diameter becomes small, the mesh sizes of the sieves are 0.15mm and 0.075 mm, and clogging occurs, measurement was carried out byreplacing sieves with JIS standard sieves having a larger diameter(diameter: 30 cm, mesh sizes of the sieves; 0.15 mm/0.075 mm).

The wt % ratio with respect to the whole crosslinked hydrogel polymer(hydrogel particles) was calculated from the weight of the crosslinkedhydrogel polymer (hydrogel particles) remaining on each sieve by thefollowing formula (8). The mesh sizes of the sieves after draining wereconverted according to the following formula (9) and the particle sizedistribution of the crosslinked hydrogel polymer (hydrogel particles)was plotted on a logarithmic probability paper. From this graph, theparticle diameter in which the residual percentage corresponds to 50 wt% was read as the weight average particle diameter (D50) of thecrosslinked hydrogel polymer (hydrogel particles).

[Mathematical Formula 10]

X(%)=(w/W)*100   (Equation 8)

[Mathematical Formula 11]

R(α) (mm)=(20/W)^(1/3) *r   (Equation 9)

Incidentally, here,

X; wt % (%) of the crosslinked hydrogel polymer (hydrogel particles)remaining on each of the sieves after being classified and drained

w; a weight (g) of the crosslinked hydrogel polymer (hydrogel particles)remaining on each of the sieves after being classified and drained

W; a total weight (g) of the crosslinked hydrogel polymer (hydrogelparticles) remaining on each of the sieves after being classified anddrained

R(α); a mesh size (mm) of a sieve in terms of a crosslinked hydrogelpolymer whose solids content is α wt %

r; a mesh size (mm) of a sieve with which a crosslinked hydrogel polymer(hydrogel particles) having swollen in a 20 wt % aqueous sodium chloridesolution is classified

(j) Weight Average Particle Diameter converted to Dried Product ofHydrogel Particles

When GelD50 is a weight average particle diameter (μm) of the hydrogelparticles,

GS is a solids content (wt %) of the hydrogel particles, and

SolidD50 is a weight average particle diameter (μm) in terms of thedried product of the hydrogel particles,

the weight average particle diameter of the hydrogel particles in termsof the dried product is defined by the following formula.

(formula) SolidD50=GelD50×(GS/100)^(1/3)

(k) Weight Average Molecular Weight

Measurement was conducted by a size exclusion chromatography (GPC) interms of polyethylene glycol under the following measurement conditions.

Measurement Conditions

Apparatus: Waters Alliance (2695) manufactured by Waters Corp.

Analysis software: Empower professional +GPC option, manufactured byWaters Corp.

Used column: TSK guard column SWXL+TSKgel G4000SWXL+G3000SWXL+G2000SWXL,manufactured by Tosoh Corporation

Detector: differential refractive index (RI) detector (manufactured byWaters Corp., Waters 2414)

Eluent: prepared by dissolving 115.6 g of sodium acetate trihydrate intoa mixture solvent of 10,999 g of water and 6,001 g of acetonitrile andadjusting the pH of the mixture to 6.0 with acetic acid

Standard substance for preparing calibration curve: polyethylene glycol[peak top molecular weights (Mp): 300,000, 200,000, 107,000, 50,000,27,700, 11,840, 6,450, 1,470, and 472]

Calibration curve: Third-order calibration curve prepared based on theMp values of the above polyethylene glycols and elution times thereof

Flow rate: 1.0 mL/min

Column temperature: 40° C.

Measurement time: 45 minutes

Amount of sample solution injected: 100 μL (eluent solution with asample concentration of 0.5 wt %)

(1) BET Specific Surface Area

The BET specific surface area of the water-absorbing resin powder of thepresent invention was measured by the following method. A high-accuracygas/vapor adsorption amount measuring apparatus (manufactured by BellJapan Inc., BELSORP-max) was used in measurement of the BET specificsurface area and a pretreatment apparatus for adsorption measurement(manufactured by Bell Japan Inc., BELSORP-vacII) was used inpretreatment.

A Pyrex (registered trademark) glass rod was put into a Pyrex(registered trademark) test tube, attached to the BET specific surfacearea measurement apparatus, and reduced-pressure degassing was performedusing a pretreatment apparatus for adsorption measurement. After apressure reached a predetermined apparatus pressure, water-absorbingresin powder was put into the Pyrex (registered trademark) test tube byvisual inspection so as to fill about 80% of the test chamber belowusing a Pyrex (registered trademark) sampling funnel for BELSORP-maxattached to the measurement apparatus. At the time, the weight of thewater-absorbing resin powder was recorded. Thereafter, the test tube wassubjected to reduced-pressure degassing using the pretreatment apparatusfor adsorption measurement. After a pressure reached a predeterminedapparatus pressure, measurement was carried out by high-accuracygas/vapor adsorption amount measuring apparatus at a liquid nitrogentemperature using krypton gas as an adsorbate. The BET specific surfacearea was obtained from the obtained adsorption isotherm by the BETtheory with use of analysis software.

Example 1

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

Into a reaction vessel constituted of a lid equipped with a thermometer,a nitrogen gas introduction pipe and a discharge hole and a tray (bottomsurface: 300 mm×220 mm, depth: 60 mm), 170 g of acrylic acid, 1,800 g ofa 37 wt % aqueous sodium acrylate solution, 0.99 g of polyethyleneglycol diacrylate (weight average molecular weight: 523), 6.688 g (0.8wt % with respect to the monomer component) of polyethylene glycol(weight average molecular weight: 2,000, manufactured by Wako PureChemical Industries, Ltd.) and 216 g of deionized water were suppliedand mixed, and the reaction vessel was immersed up to a height of 10 mmfrom the bottom of the tray in a water bath set at 20° C.

Nitrogen gas was introduced into this aqueous solution and degassed for20 minutes. After confirming that this solution became 20° C., 6.61 g ofa 20 wt % aqueous sodium persulfate solution and 6.33 g of a 0.1 wt %aqueous L-ascorbic acid solution were added thereto under a nitrogenflow atmosphere and mixed under stirring. The monomer concentration was38 wt %. After 1 minute, polymerization was initiated, and thetemperature of the reaction system at this time was 20° C. Afterinitiation of polymerization, the polymerization system was not stirred,and subsequently the reaction vessel was immersed in the water bath setat 20° C. so as to be cooled. After 17 minutes, the polymerizationsystem showed the maximum reaching temperature of 89° C. Thereafter, thetemperature of the water bath was adjusted to 70° C. and then thepolymerization reaction was performed for 20 minutes to thereby obtain acrosslinked hydrogel polymer (GK1).

(Gel-Crushing Step)

The obtained crosslinked hydrogel polymer (GK1) was cut into blocks, putinto UNIPACK (manufactured by SEISANNIPPONSHA LTD.), and left to standstill in a thermostat for 1 hour at a constant temperature of 60° C. Thecrosslinked hydrogel polymer (GK1), the temperature of which wasmaintained at a constant temperature of 60° C., was allowed to passtwice through a meat chopper (manufactured by REMACOM, model:HL-G22SN)having a die plate with a aperture diameter of 3.5 mm andwarmed to 60° C. by using a sheet heater, thereby obtaining hydrogelparticles (also referred to as “crosslinked hydrogel polymer crushedproduct”) (GKF1).

The rotation speed of the screw axis of the meat chopper was set to 210rpm, the crosslinked hydrogel polymer (GK1) was supplied at 360 g/min,and then the obtained gel-crushed product was also supplied similarly at360 g/min and was allowed to pass twice through the meat chopper.

The physical properties of the crosslinked hydrogel polymer crushedproduct (GKF1) are presented in the following table. The followingcrosslinked hydrogel polymer crushed product (GKF) is presentedsimilarly in the following table.

(Steps for Drying, Pulverizing and Classifying)

The obtained crosslinked hydrogel polymer crushed product (GKF1) wasdried at 160° C. for 45 minutes by a hot air drier to obtain a driedproduct. Thereafter, the dried product was crushed by a roll mill(manufactured by Inokuchi Giken Limited Company), and was sifted withsieves having mesh sizes of 850 μm, 600 μm, 500 μm, 300 μm, and 150 μm.Then, blending was carried out such that particles passing through asieve having a mesh size of 850 μm and not passing through a sievehaving a mesh size of 600 μm were 3 wt %, particles passing through asieve having a mesh size of 600 μm and not passing through a sievehaving a mesh size of 500 μm were 10 wt %, particles passing through asieve having a mesh size of 500 μm and not passing through a sievehaving a mesh size of 300 μm were 54 wt %, particles passing through asieve having a mesh size of 300 μm and not passing through a sievehaving a mesh size of 150 μm were 31 wt %, and particles passing througha sieve having a mesh size of 150 μm and not passing through a sievehaving a mesh size of 45 μm were 2 wt %. Thereby, water-absorbing resinpowder (B1) was obtained. Incidentally, the blending was similarlyperformed except Example 15.

The weight average particle diameter D50, the logarithmic standarddeviation, and CRC of the water-absorbing resin powder (B1) arepresented in the following table. The water-absorbing resin powder (B)described below is presented similarly in the following table.

(Surface Crosslinking Step)

A surface crosslinking agent solution containing 0.025 parts by weightof ethylene glycol diglycidyl ether, 0.3 parts by weight of ethylenecarbonate, 0.5 parts by weight of propylene glycol, and 2.0 parts byweight of deionized water was mixed with 100 parts by weight of thewater-absorbing resin powder (B1) by spraying. The above-describedmixture was subjected to the heat treatment at 200° C. for 35 minutes toobtain surface crosslinked water-absorbing resin particles (S1).

(Step for Adding Liquid Permeability Enhancer)

1 part by weight of a 1 wt % aqueous pentasodium diethylenetriaminepentaacetate (DTPA) solution with respect to 100 parts by weight of thesurface crosslinked water-absorbing resin particles (S1) was added as achelating agent under stirring and mixed for 1 minute. When thechelating agent is contained, urine resistance is improved.

Subsequently, the resultant mixture was left to stand in a hot air drierset at 60° C. for 30 minutes and was then allowed to pass through a wiremesh having a mesh size of 850 μm, and then 0.6 parts by weight of fumedsilica (Aerosil 200, manufactured by NIPPON AEROSIL Co., Ltd.) was mixedtherewith. For mixing, 30 g of the surface crosslinked water-absorbingresin particles (S1) was put with fumed silica into a mayonnaise jarhaving a volume of 225 ml and shaken for 3 minutes by using a paintshaker to obtain a particulate water-absorbing agent (EX-1). Theperformance of the obtained particulate water-absorbing agent (EX-1) isdescribed below. The same applies hereafter.

Example 2

Water-absorbing resin powder (B2), surface-treated water-absorbing resinparticles (S2), and a particulate water-absorbing agent (EX-2) wereobtained by performing the operation similar to that of Example 1,except that the amount of polyethylene glycol used was changed to 3.344g (0.4 wt % with respect to the monomer component).

Example 3

Water-absorbing resin powder (B3), surface-treated water-absorbing resinparticles (S3), and a particulate water-absorbing agent (EX-3) wereobtained by performing the operation similar to that of Example 1,except that the amount of polyethylene glycol used was changed to 10.03g (1.2 wt % with respect to the monomer component).

Example 4

Water-absorbing resin powder (B4), surface-treated water-absorbing resinparticles (S4), and a particulate water-absorbing agent (EX-4) wereobtained by performing the operation similar to that of Example 1,except that the weight average molecular weight of polyethylene glycolused was changed to 400.

Example 5

Water-absorbing resin powder (B5), surface-treated water-absorbing resinparticles (S5), a particulate water-absorbing agent (EX-5) were obtainedby performing the operation similar to that of Example 1, except thatthe weight average molecular weight of polyethylene glycol used waschanged to 20,000.

Example 6

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

Into a reaction vessel constituted of a lid equipped with a thermometer,a nitrogen gas introduction pipe and a discharge hole, and a tray(bottom surface: 300 mm×220 mm, depth: 60 mm), 170 g of acrylic acid,1,800 g of 37 wt % aqueous sodium acrylate solution, 0.99 g ofpolyethylene glycol diacrylate (average molecular weight: 523), and 216g of deionized water were supplied and mixed, and the reaction vesselwas immersed up to a height of 10 mm from the bottom of the tray in awater bath set at 20° C. Nitrogen gas was introduced into this aqueoussolution and degassed for 20 minutes. After confirming that thissolution became 20° C., 6.61 g of a 20 wt % aqueous sodium persulfatesolution and 6.33 g of a 0.1 wt % aqueous L-ascorbic acid solution wereadded thereto under a nitrogen flow atmosphere and mixed under stirring.The monomer concentration was 38 wt %. After 1 minute, polymerizationwas initiated, and the temperature of the reaction system at this timewas 20° C. After initiation of polymerization, the polymerization systemwas not stirred, and subsequently the reaction vessel was immersed inthe water bath set at 20° C. so as to be cooled.

After 17 minutes, the polymerization system showed the maximum reachingtemperature of 89° C. Thereafter, the temperature of the water bath wasadjusted to 70° C. and then the polymerization reaction was performedfor 20 minutes to thereby obtain a crosslinked hydrogel polymer (GK6).The obtained crosslinked hydrogel polymer (GK6) was cut into blocks.

The obtained crosslinked hydrogel polymer (GK6) was cut into blocks, putinto UNIPACK (manufactured by SEISANNIPPONSHA LTD.), and left to standstill in a thermostat for 1 hour at a constant temperature of 60° C. Inthe crosslinked hydrogel polymer (GK6), the temperature of which wasmaintained at a constant temperature of 60° C., 66.9 g of a methanolsolution containing 10 wt % of polyethylene glycol (weight averagemolecular weight: 2,000, manufactured by Wako Pure Chemical Industries,Ltd.) was sprinkled uniformly on the surface of 2,200 g of theblock-shaped crosslinked hydrogel polymer (GK6), and was allowed to passtwice through a meat chopper (manufactured by REMACOM, model: HL-G22SN)having a die plate with a aperture diameter of 3.5 mm and warmed to 60°C. by using a sheet heater, thereby obtaining a crosslinked hydrogelpolymer crushed product (GKF6).

The rotation speed of the screw axis of the meat chopper was set to 210rpm, the crosslinked hydrogel polymer (GK6) was supplied at 360 g/min,and then the obtained gel-crushed product was also supplied similarly at360 g/min and was allowed to pass twice through the meat chopper.

(Steps for Drying, Pulverizing and Classifying)

The obtained crosslinked hydrogel polymer crushed product (GKF6) wasdried at 160° C. for 45 minutes by a hot air drier to obtain a driedproduct. Thereafter, the dried product was crushed by a roll mill(manufactured by Inokuchi Giken Limited Company), and was sifted withsieves having mesh sizes of 850 μm, 600 μm, 500 μm, 300 μm, and 150 μm.Then, blending was carried out to obtain water-absorbing resin powder(B6). The performance of the obtained water-absorbing resin powder (B6)is presented in Table 1.

(Surface Crosslinking Step)

A surface crosslinking agent solution containing 0.025 parts by weightof ethylene glycol diglycidyl ether, 0.3 parts by weight of ethylenecarbonate, 0.5 parts by weight of propylene glycol, and 2.0 parts byweight of deionized water was mixed with 100 parts by weight of thewater-absorbing resin powder (B6). The above-described mixture wassubjected to the heat treatment at 200° C. for 35 minutes to obtainsurface crosslinked water-absorbing resin particles (S6).

(Step for Adding Liquid Permeability Enhancer)

1 part by weight of a 1 wt % aqueous DTPA solution with respect to 100parts by weight of the surface crosslinked water-absorbing resinparticles (S6) was added under stirring and mixed for 1 minute.Subsequently, the resultant mixture was left to stand in a hot air drierset at 60° C. for 30 minutes and was then allowed to pass through a wiremesh having a mesh size of 850 μm, and then 0.6 parts by weight ofhydrotalcite (DHT-6, manufactured by Kyowa Chemical Industry Co., Ltd.)

was mixed therewith. For mixing, 30 g of the surface crosslinkedwater-absorbing resin particles (S6) was put with hydrotalcite into amayonnaise jar having a volume of 225 ml and shaken for 3 minutes byusing a paint shaker to obtain a particulate water-absorbing agent(EX-6). The performance of the obtained particulate water-absorbingagent (EX-6) is presented in the following table.

Example 7

1 part by weight of a 1 wt % aqueous DTPA solution with respect to 100parts by weight of the surface crosslinked water-absorbing resinparticles (S1) in Example 1 was added under stirring and mixed for 1minute. Further, a solution containing 1.17 parts by weight of a 27.5 wt% aqueous aluminum sulfate solution (8 wt % in terms of aluminum oxide),0.196 parts by weight of a 60 wt % aqueous sodium lactate solution, and0.029 parts by weight of propylene glycol was added thereto and mixedfor 1 minute. Thereafter, the resultant mixture was left to stand in ahot air drier for 30 minutes and was then allowed to pass through a wiremesh having a mesh size of 850 μm to obtain a particulatewater-absorbing agent (EX-7). The performance of the obtainedparticulate water-absorbing agent (EX-7) is presented in the followingtable.

Example 8

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK8) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g.

(Gel-Crushing Step)

The obtained crosslinked hydrogel polymer (GK8) was cut into blocks, putinto UNIPACK (manufactured by SEISANNIPPONSHA LTD.), and left to standstill in a thermostat for 1 hour to at a constant temperature of 60° C.The crosslinked hydrogel polymer (GK8), the temperature of which wasmaintained at a constant temperature of 60° C., was supplied to a screwextruder, which has been warmed to 60° C. by using a sheet heater, andthen gel-crushed. As the screw extruder, a meat chopper, which isprovided with a porous plate (diameter: 100 mm, pore diameter: 3.2 mm,the number of pores: 316, opening ratio: 32.3%, thickness: 10 mm) at thetip end and in which the screw axis has an outer diameter of 86 mm andthe internal diameter of the casing is 88 mm, was used.

The rotation speed of the screw axis of the meat chopper was set to 126rpm, and the crosslinked hydrogel polymer (GK8) was supplied at 1,680g/min and allowed to pass once, thereby obtaining a crosslinked hydrogelpolymer crushed product (GKF8).

At this time, the gel grinding energy GGE (1) was 174.8 J/g and the gelgrinding energy GGE (2) was 40.4 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B8) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF8) was dried at 185° C.for 30 minutes by a hot air drier.

(Surface Crosslinking Step) and (Step for Adding Liquid PermeabilityEnhancer)

The obtained water-absorbing resin powder (B8) was subjected to theoperation similar to that of Example 1 to obtain surface crosslinkedwater-absorbing resin particles (S8) and a particulate water-absorbingagent (EX-8). The performance of the obtained particulatewater-absorbing agent (EX-8) is presented in the following table.

Example 9

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK9) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF9) was obtained byperforming the operation similar to that of Example 8, except that 6.688g (0.8 wt % with respect to the monomer concentration) of polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was supplied at 20.5 g/min as a 20 wt %aqueous solution and the obtained crosslinked hydrogel polymer (GK9) wassupplied at 1680 g/min.

At this time, the gel grinding energy GGE (1) was 175.7 J/g and the gelgrinding energy GGE (2) was 41.3 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B9) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF9) was dried at 185° C.for 30 minutes by a hot air drier.

(Surface Crosslinking Step) and (Step for Adding Liquid PermeabilityEnhancer)

The obtained water-absorbing resin powder (B9) was subjected to theoperation similar to that of Example 1 to obtain surface crosslinkedwater-absorbing resin particles (S9) and a particulate water-absorbingagent (EX-9). The performance of the obtained particulatewater-absorbing agent (EX-9) is presented in the following table.

In Example 9 described above, the adhesion controlling agent is notadded during polymerization but the adhesion controlling agent is addedwhile the meat chopper is operated (during gel-crushing).

Example 10

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK10) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g.

(Gel-Crushing Step)

The obtained crosslinked hydrogel polymer (GK10) was subjected to theoperation similar to that of Example 8 to obtain a crosslinked hydrogelpolymer crushed product (GKF11).

At this time, the gel grinding energy GGE (1) was 174.8 J/g and the gelgrinding energy GGE (2) was 40.4 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B10) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF10) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S10) were obtained byperforming the operation similar to that of Example 1, except that theheat treatment time of the water-absorbing resin powder (B10) was set to45 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S10) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-10). The performance of the obtainedparticulate water-absorbing agent (EX-10) is presented in the followingtable.

In Example 11, the thermal treatment time of the surface crosslinkingstep was set to be relatively longer. According to this, the effect ofimproving FGBP is achieved.

Example 11

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A monomer solution containing 380 g of acrylic acid, 158 g of a 48 wt %aqueous sodium hydroxide solution, 1.53 g of polyethylene glycoldiacrylate (weight average molecular weight: 523), 3.74 g (0.8 wt % withrespect to the monomer component) of polyethylene glycol (weight averagemolecular weight: 2,000, manufactured by Wako Pure Chemical Industries,Ltd.), and 23.4 g of a 0.1 wt % aqueous trisodium diethylene triaminepentaacetate solution was prepared. Then, 162 g of a 48 wt % aqueoussodium hydroxide solution was added to the monomer solution adjusted to45° C. with stirred. The temperature of the mixture was increased to 80°C. by neutralization heat at this time. Further, 18.58 g of a 4 wt %aqueous sodium persulfate solution was added and the mixture was flowedon Teflon (registered trademark) sheet (a reaction vessel having a sizeof 30 cm×30 cm whose four sides are surrounded by weirs having a heightof 1.5 cm) whose the bottom surface has been already warmed to 50° C.,in an ambient temperature of 60° C. so as to perform polymerization.After 1 minute from the addition of the aqueous sodium persulfatesolution, the polymerization system showed the maximum reachingtemperature of 105° C. After further 4 minutes, the obtained polymer wastaken out to obtain a crosslinked hydrogel polymer (GK11).

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF11) was obtained byperforming the operation similar to that of Example 8, except that therotation speed of the screw axis was set to 130 rpm and the crosslinkedhydrogel polymer (GK11) was supplied at 4,640 g/min to the meat chopper.

At this time, the gel grinding energy GGE(1) was 69.4 J/g and the gelgrinding energy GGE (2) was 24.0 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B11) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF11) was dried at 190°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S11) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B11) was set to 25 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S11) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-11). The performance of the obtainedparticulate water-absorbing agent (EX-11) is presented in the followingtable.

Example 12

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK12) was obtained by performing theoperation similar to that of Example 11, except that the amount ofpolyethylene glycol diacrylate was changed to 1.39 g and 1.17 g (0.075wt % as an effective ingredient with respect to the monomer component)of AMPHITOL 20BS (Kao Corporation, effective ingredient: 30 wt %) wasused instead of polyethylene glycol (weight average molecular weight:2,000, manufactured by Wako Pure Chemical Industries, Ltd.).

(Gel-Crushing Step)

As the screw extruder, a meat chopper, which is provided with a porousplate (diameter: 100 mm, pore diameter: 6.4 mm, the number of pores: 83,opening ratio: 34.0%, thickness: 10 mm) at the tip end and in which thescrew axis has an outer diameter of 86 mm and of which the internaldiameter of the casing is 88 mm, was used. A crosslinked hydrogelpolymer crushed product (GKF12) was obtained by performing the operationsimilar to that of Example 8, except that the meat chopper was used andthe crosslinked hydrogel polymer (GK12) was supplied at 4,640 g/min andthe rotation speed of the screw axis of 130 rpm to the meat chopper.

At this time, the gel grinding energy GGE (1) was 57.5 J/g and the gelgrinding energy GGE (2) was 13.3 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B12) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF12) was dried at 190°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S12) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B12) was set to 30 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S12) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-12). The performance of the obtainedparticulate water-absorbing agent (EX-12) is presented in the followingtable.

In Example 12, the particle diameter of the crosslinked hydrogel polymercrushed product (GKF12) is increased by lowering GGE.

Example 13

(Preparation and Polymerization Steps of (Meth) Acrylic Acid(Salt)-Based Aqueous Monomer Solution)

A crosslinked hydrogel polymer (GK13) was obtained by performing theoperation similar to that of Example 11, except that 1.17 g (0.075 wt %as an effective ingredient with respect to the monomer component) ofAMPHITOL 20BS (manufactured by Kao Corporation, effective ingredient: 30wt %) was used instead of polyethylene glycol (weight average molecularweight: 2,000, manufactured by Wako Pure Chemical Industries, Ltd.).

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF13) was obtained byperforming the operation similar to that of Example 8, except that therotation speed of the screw axis was set to 130 rpm and the crosslinkedhydrogel polymer (GK13) was supplied at 4,640 g/min to the meat chopper.

At this time, the gel grinding energy GGE (1) was 65.5 J/g and the gelgrinding energy GGE (2) was 24.4 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B13) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF13) was dried at 190°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S13) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B13) was set to 25 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S13) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-13). The performance of the obtainedparticulate water-absorbing agent (EX-13) is presented in the followingtable.

This Example has excellent results as compared to other Examples. Thereason for this is assumed that the polymerization method is short-termpolymerization using neutralization heat, the molecular weight of themain chain is large, the molecular weight distribution is also narrow,the physical properties are improved, and the adhesion controlling agentis particularly suitable.

Example 14

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK14) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and 2.090 g (0.075wt % as an effective ingredient with respect to the monomer component)of AMPHITOL 20BS (manufactured by Kao Corporation, effective ingredient:30 wt %) was used instead of polyethylene glycol (weight averagemolecular weight: 2,000, manufactured by Wako Pure Chemical Industries,Ltd.).

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF14) was obtained byperforming the operation similar to that of Example 8, except that thenumber of revolutions of the screw axis was set to 225 rpm.

At this time, the gel grinding energy GGE (1) was 182.2 J/g and the gelgrinding energy GGE (2) was 58.3 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B14) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF14) was dried at 185°C. for 30 minutes by a hot air drier and blending was carried out suchthat particles passing through a sieve having a mesh size of 850 μm andnot passing through a sieve having a mesh size of 600 μm were 12 wt %,particles passing through a sieve having a mesh size of 600 μm and notpassing through a sieve having a mesh size of 500 μm were 25 wt %,particles passing through a sieve having a mesh size of 500 μm and notpassing through a sieve having a mesh size of 300 μm were 41 wt %,particles passing through a sieve having a mesh size of 300 μm and notpassing through a sieve having a mesh size of 150 μm were 21 wt %, andparticles passing through a sieve having a mesh size of 150 μm and notpassing through a sieve having a mesh size of 45 μm were 1 wt %.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S14) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B14) was set to 25 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S14) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-14). The performance of the obtainedparticulate water-absorbing agent (EX-14) is presented in the followingtable.

Example 15

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK15) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and 2.090 g (0.075wt % as an effective ingredient with respect to the monomer component)of AMPHITOL 20BS (manufactured by Kao Corporation, effective ingredient:30 wt %) was used instead of polyethylene glycol (weight averagemolecular weight: 2,000, manufactured by Wako Pure Chemical Industries,Ltd.).

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF15) was obtained byperforming the operation similar to that of Example 8, except that therotation speed of the screw axis was set to 225 rpm.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B15) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF15) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S15) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B15) was set to 25 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S15) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-15). The performance of the obtainedparticulate water-absorbing agent (EX-15) is presented in the followingtable.

Example 16

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK16) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

The obtained crosslinked hydrogel polymer (GK16) was cut into blocks,put into UNIPACK (manufactured by SEISANNIPPONSHA LTD.), and left tostand still in a thermostat for 1 hour at a constant temperature of 60°C. 62.7 g (0.075 wt % as an effective ingredient with respect to the rawmaterial monomer component amount of the step (i)) of a methanolsolution containing 1 wt % of AMPHITOL 20BS (manufactured by KaoCorporation, effective ingredient: 30 wt %) was sprinkled uniformly onthe surface of the crosslinked hydrogel polymer (GK16) maintained at aconstant temperature of 60° C., and was allowed to pass twice through ameat chopper (manufactured by Iizuka Corporation, model: ROYAL, type:VR-400K) having a die plate with an aperture diameter of 4.7 mm andwarmed to 60° C. by using a sheet heater, thereby obtaining acrosslinked hydrogel polymer crushed product (GKF16).

The rotation speed of the screw axis of the meat chopper was set to 170rpm, a crosslinked hydrogel polymer (GK17) was supplied at 150 g/min,and then the obtained gel-crushed product was also supplied similarly at150 g/min and was allowed to pass twice through the meat chopper.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B16) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF16) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S16) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B16) was set to 30 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S16) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-16). The performance of the obtainedparticulate water-absorbing agent (EX-16) is presented in the followingtable.

Example 17

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK17) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF17) was obtained byperforming the operation similar to that of Example 16, except that 62.7g (0.075 wt % as an effective ingredient with respect to the rawmaterial monomer component amount of the step (i)) of a methanolsolution containing 1 wt % of AMPHITOL 20HD (manufactured by KaoCorporation, effective ingredient: 30 wt %) was sprinkled uniformly onthe surface of the block obtained by cutting the obtained crosslinkedhydrogel polymer (GK17).

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B17) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF17) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S17) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B17) was set to 30 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S17) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-17). The performance of the obtainedparticulate water-absorbing agent (EX-17) is presented in the followingtable.

Example 18

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK18) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF18) was obtained byperforming the operation similar to that of Example 17, except that 62.7g (0.075 wt % as an effective ingredient with respect to the rawmaterial monomer component amount of the step (i)) of a methanolsolution containing 1 wt % of AMPHITOL 20N (manufactured by KaoCorporation, effective ingredient: 30 wt %) was sprinkled uniformly onthe surface of the block obtained by cutting the obtained crosslinkedhydrogel polymer (GK18).

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B18) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF18) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S18) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B18) was set to 30 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S18) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-18). The performance of the obtainedparticulate water-absorbing agent (EX-18) is presented in the followingtable.

Example 19

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK19) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF19) was obtained byperforming the operation similar to that of Example 16, except that 55.7g (0.050 wt % as an effective ingredient with respect to the rawmaterial monomer component amount of the step (i)) of a methanolsolution containing 0.75 wt % of AMIET 105A (manufactured by KaoCorporation, effective ingredient: 100 wt %) was sprinkled uniformly onthe surface of the block obtained by cutting the obtained crosslinkedhydrogel polymer (GK19).

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B19) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF19) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S19) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B19) was set to 30 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S19) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-19). The performance of the obtainedparticulate water-absorbing agent (EX-19) is presented in the followingtable.

Example 20

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK20) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF20) was obtained byperforming the operation similar to that of Example 16, except that 62.7g (0.075 wt % as an effective ingredient with respect to the rawmaterial monomer component amount of the step (i)) of an isopropylalcohol solution containing 1 wt % of EMAL 20C (manufactured by KaoCorporation, effective ingredient: 25 wt %) was sprinkled uniformly onthe surface of the block obtained by cutting the obtained crosslinkedhydrogel polymer (GK20).

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B20) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF20) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S20) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B20) was set to 30 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S20) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-20). The performance of the obtainedparticulate water-absorbing agent (EX-20) is presented in the followingtable.

Example 21

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK21) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF21) was obtained byperforming the operation similar to that of Example 16, except that 62.7g (0.075 wt % as an effective ingredient with respect to the rawmaterial monomer component amount of the step (i)) of a methanolsolution containing 1 wt % of ACETAMIN 24 (manufactured by KaoCorporation, effective ingredient: 98 wt %) was sprinkled uniformly onthe surface of the block obtained by cutting the obtained crosslinkedhydrogel polymer (GK21).

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B21) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF21) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S21) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B21) was set to 20 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S21) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-21). The performance of the obtainedparticulate water-absorbing agent (EX-21) is presented in the followingtable.

Example 22

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK22) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF22) was obtained byperforming the operation similar to that of Example 16, except that 62.7g (0.075 wt % as an effective ingredient with respect to the rawmaterial monomer component amount of the step (i)) of a methanolsolution containing 1 wt % of Adeka Pluronic L-44 (manufactured by ADEKACORPORATION, effective ingredient: 100 wt %) was sprinkled uniformly onthe surface of the block obtained by cutting the obtained crosslinkedhydrogel polymer (GK22).

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B22) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF22) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S22) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B22) was set to 30 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S22) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-22). The performance of the obtainedparticulate water-absorbing agent (EX-22) is presented in the followingtable.

Example 23

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK23) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF23) was obtained byperforming the operation similar to that of Example 16, except that 62.7g (0.075 wt % as an effective ingredient with respect to the rawmaterial monomer component amount of the step (i)) of a methanolsolution containing 1 wt % of EMULGEN 430 (manufactured by KaoCorporation, effective ingredient: 100 wt %) was sprinkled uniformly onthe surface of the block obtained by cutting the obtained crosslinkedhydrogel polymer (GK23).

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B23) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF23) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S23) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B23) was set to 30 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S23) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-23). The performance of the obtainedparticulate water-absorbing agent (EX-23) is presented in the followingtable.

Example 24

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK24) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF24) was obtained byperforming the operation similar to that of Example 16, except that 62.7g (0.075 wt % as an effective ingredient with respect to the rawmaterial monomer component amount of the step (i)) of a methanolsolution containing 1 wt % of PELEX SS-L (manufactured by KaoCorporation, effective ingredient: 50 wt %) was sprinkled uniformly onthe surface of the block obtained by cutting the obtained crosslinkedhydrogel polymer (GK24).

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B24) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF24) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S24) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B24) was set to 30 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S24) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-24). The performance of the obtainedparticulate water-absorbing agent (EX-24) is presented in the followingtable.

Example 25

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK25) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF25) was obtained byperforming the operation similar to that of Example 16, except that 66.9g (0.8 wt % as an effective ingredient with respect to the raw materialmonomer component amount of the step (i)) of a methanol solutioncontaining 10 wt % of Denacol EX-861 (manufactured by Nagase ChemteXCorporation, effective ingredient: 100 wt %) was sprinkled uniformly onthe surface of the block obtained by cutting the obtained crosslinkedhydrogel polymer (GK25).

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B25) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF25) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S25) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B25) was set to 20 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S25) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-25). The performance of the obtainedparticulate water-absorbing agent (EX-25) is presented in the followingtable.

Example 26

(Gel Polymerization Step)

A crosslinked hydrogel polymer (GK26) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF26) was obtained byperforming the operation similar to that of Example 16, except that 62.7g (0.050 wt % as an effective ingredient with respect to the rawmaterial monomer component amount of the step (i)) of a methanolsolution containing 1 wt % of RHEODOL TW-S120V (manufactured by KaoCorporation, effective ingredient: 100 wt %, catalog HLB value: 14.9)was sprinkled uniformly on the surface of the block obtained by cuttingthe obtained crosslinked hydrogel polymer (GK26).

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B26) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF26) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S26) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B26) was set to 20 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S26) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-26). The performance of the obtainedparticulate water-absorbing agent (EX-26) is presented in the followingtable.

Example 27

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK27) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and polyethyleneglycol (weight average molecular weight: 2,000, manufactured by WakoPure Chemical Industries, Ltd.) was not used.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF27) was obtained byperforming the operation similar to that of Example 16, except that 62.7g (0.050 wt % as an effective ingredient with respect to the rawmaterial monomer component amount of the step (i)) of a methanolsolution containing 1 wt % of KF-354L (manufactured by Shin-EtsuChemical Co., Ltd., effective ingredient: 100 wt %, catalog HLB value:16) was sprinkled uniformly on the surface of the block obtained bycutting the obtained crosslinked hydrogel polymer (GK27).

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B27) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF27) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S27) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B27) was set to 20 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S27) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-27). The performance of the obtainedparticulate water-absorbing agent (EX-27) is presented in the followingtable.

Example 28

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK28) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF28) was obtained byperforming the operation similar to that of Example 8, except that asthe screw extruder, a meat chopper, which is provided with a porousplate (diameter: 100 mm, pore diameter: 8.0 mm, the number of pores: 54,opening ratio: 34.5%, thickness: 10 mm) at the tip end and in which thescrew axis has an outer diameter of 86 mm and of which the internaldiameter of the casing is 88 mm, was used.

At this time, the gel grinding energy GGE (1) was 108.0 J/g and the gelgrinding energy GGE (2) was 14.2 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B28) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF28) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step) and (Step for Adding Liquid PermeabilityEnhancer)

The water-absorbing resin powder (B28) was subjected to the operationsimilar to that of Example 1 to obtain surface crosslinkedwater-absorbing resin particles (S28) and a particulate water-absorbingagent (EX-28). The performance of the obtained particulatewater-absorbing agent (EX-28) is presented in the following table.

Example 29

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK29) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and 2.090 g (0.075wt % as an effective ingredient with respect to the monomer component)of AMPHITOL 20BS (manufactured by Kao Corporation, effective ingredient:30 wt %) was used instead of polyethylene glycol (weight averagemolecular weight: 2,000, manufactured by Wako Pure Chemical Industries,Ltd.).

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product

(GKF29) was obtained by performing the operation similar to that ofExample 8, except that the rotation speed of the screw axis was set to225 rpm.

At this time, the gel grinding energy GGE(1) was 182.2 J/g and the gelgrinding energy GGE (2) was 58.3 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B29) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF29) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S29) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B29) was set to 40 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S29) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-29). The performance of the obtainedparticulate water-absorbing agent (EX-29) is presented in the followingtable.

Example 30

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK30) was obtained by performing theoperation similar to that of Example 1, except that the amount ofpolyethylene glycol diacrylate was changed to 1.73 g and 2.090 g (0.075wt % as an effective ingredient with respect to the monomer component)of AMPHITOL 20BS (manufactured by Kao Corporation, effective ingredient:30 wt %) was used instead of polyethylene glycol (weight averagemolecular weight: 2,000, manufactured by Wako Pure Chemical Industries,Ltd.).

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF30) was obtained byperforming the operation similar to that of Example 8, except that therotation speed of the screw axis was set to 225 rpm.

At this time, the gel grinding energy GGE (1) was 182.2 J/g and the gelgrinding energy GGE (2) was 58.3 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B30) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF30) was dried at 185°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S30) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B30) was set to 20 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S30) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-30). The performance of the obtainedparticulate water-absorbing agent (EX-30) is presented in the followingtable.

Example 31

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK31) was obtained by performing theoperation similar to that of Example 11.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF31) was obtained byperforming the operation similar to that of Example 8, except that asthe screw extruder, a meat chopper, which is provided with a porousplate (diameter: 100 mm, pore diameter: 6.4 mm, the number of pores: 83,opening ratio: 34.0%, thickness: 10 mm) at the tip end and in which thescrew axis has an outer diameter of 86 mm and of which the internaldiameter of the casing is 88 mm, was used, the rotation speed of thescrew axis was set to 130 rpm, and a crosslinked hydrogel polymer (GK32)was supplied at 4,640 g/min to the meat chopper.

At this time, the gel grinding energy GGE (1) was 56.4 J/g and the gelgrinding energy GGE (2) was 13.1 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B31) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF31) was dried at 190°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S31) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B31) was set to 20 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S31) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-31). The performance of the obtainedparticulate water-absorbing agent (EX-31) is presented in the followingtable.

Example 32

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (GK32) was obtained by performing theoperation similar to that of Example 13.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (GKF32) was obtained byperforming the operation similar to that of Example 8, except that asthe screw extruder, a meat chopper, which is provided with a porousplate (diameter: 100 mm, pore diameter: 6.4 mm, the number of pores: 83,opening ratio: 34.0%, thickness: 10 mm) at the tip end and in which thescrew axis has an outer diameter of 86 mm and of which the internaldiameter of the casing is 88 mm, was used, the rotation speed of thescrew axis was set to 130 rpm, and a crosslinked hydrogel polymer (GK33)was supplied at 4,640 g/min to the meat chopper.

At this time, the gel grinding energy GGE (1) was 56.7 J/g and the gelgrinding energy GGE (2) was 12.6 J/g.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (B32) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF32) was dried at 190°C. for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (S32) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(B32) was set to 20 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (S32) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (EX-32). The performance of the obtainedparticulate water-absorbing agent (EX-32) is presented in the followingtable.

Comparative Example 1

Hydrogel particles (CGKF-1), water-absorbing resin powder (CB-1),surface-treated water-absorbing resin particles (CS1), and a particulatewater-absorbing agent (CEX-1) were obtained by performing the treatmentsimilar to that of Example 1, except that the die aperture diameter ofthe meat chopper used in the gel-crushing step was changed from 3.5 mmto 9.0 mm in Example 1.

The performances of the obtained hydrogel particles (CGKF-1),water-absorbing resin powder (CB-1), particulate water-absorbing agent(CEX-1) are presented in the following table.

Comparative Example 2

The S1 obtained in Example 1 was used as a comparative water-absorbingagent (CEX-2). The performance of the obtained particulatewater-absorbing agent (CEX-2) is presented in the following table.

Comparative Example 3

Water-absorbing resin powder (CB-3) having a granulated form and acomparative water-absorbing agent (CEX-3) were obtained by performingthe operation similar to that of Example 1 of Application No.PCT/JP2015/56110. The performances of the obtained water-absorbing resinpowder (CB-3) and particulate water-absorbing agent (CEX-3) arepresented in the following table.

Comparative Example 4

Water-absorbing resin powder (CB-4) having a granulated form and acomparative water-absorbing agent (CEX-4) were obtained by performingthe operation similar to that of Example 2 of Application No.PCT/JP2015/56110. The performances of the obtained water-absorbing resinpowder (CB-4) and particulate water-absorbing agent (CEX-4) arepresented in the following table.

Comparative Example 5

Hydrogel particles (CGKF-5), water-absorbing resin powder (CB-5), and acomparative particulate water-absorbing agent (CEX-5) were prepared bythe method described in Example 6 of WO 2011/126079 A. The performancesof the obtained hydrogel particles (CGKF-5), water-absorbing resinpowder (CB-5), water-absorbing resin powder (CEX-5), and particulatewater-absorbing agent (CEX-6) are presented in the following table.

Comparative Example 6

Hydrogel particles (CGKF-6), water-absorbing resin powder (CB-6), and acomparative water-absorbing agent (CEX-6) were obtained by performingthe operation similar to that of Example 13 of WO 2015/030130 A. Theperformances of the obtained hydrogel particles (CGKF-6),water-absorbing resin powder (CB-6), and particulate water-absorbingagent (CEX-6) are presented in the following table.

Comparative Example 7

Hydrogel particles (CGKF-6) and a comparative water-absorbing agent(CEX-7) were obtained by performing the operation similar to that ofExample 6 of WO 2008/096713 A. The performances of the obtained hydrogelparticles (CGKF-6) and particulate water-absorbing agent (CEX-7) arepresented in the following table.

Comparative Example 8

(Preparation and Polymerization Steps of (Meth)Acrylic Acid (Salt)-BasedAqueous Monomer Solution)

A crosslinked hydrogel polymer (CGK8) was obtained by performing theoperation similar to that of Example 1, except that 0.418 g (0.05 wt %with respect to the monomer component) of sodiumdi(2-ethylhexyl)sulfosuccinate was used instead of polyethylene glycol.

(Gel-Crushing Step)

A crosslinked hydrogel polymer crushed product (CGKF8) was obtained byperforming the operation similar to that of Example 1.

(Steps for Drying, Pulverizing and Classifying)

Water-absorbing resin powder (CB8) was obtained by performing theoperation similar to that of Example 1, except that the obtainedcrosslinked hydrogel polymer crushed product (GKF8) was dried at 185° C.for 30 minutes by a hot air drier.

(Surface Crosslinking Step)

Surface-treated water-absorbing resin particles (CS8) were obtained byperforming the operation similar to that of Example 1, except that theheat temperature treatment time of the water-absorbing resin powder(CB8) was set to 60 minutes.

(Step for Adding Liquid Permeability Enhancer)

The surface-treated water-absorbing resin particles (CS8) were subjectedto the operation similar to that of Example 1 to obtain a particulatewater-absorbing agent (CEX-8). The performance of the obtainedparticulate water-absorbing agent (CEX-8) is presented in the followingtable.

TABLE 1 Additive Amount (wt %/monomer Type weight) Example 1Polyethylene glycol (Mw2000) 0.8 Example 2 Polyethylene glycol (Mw2000)0.4 Example 3 Polyethylene glycol (Mw2000) 1.2 Example 4 Polyethyleneglycol (Mw400) 0.8 Example 5 Polyethylene glycol 0.8 (Mw20000) Example 6Polyethylene glycol (Mw2000) 0.8 Example 7 Polyethylene glycol (Mw2000)0.8 Example 8 Polyethylene glycol (Mw2000) 0.8 Example 9 Polyethyleneglycol (Mw2000) 0.8 Example 10 Polyethylene glycol (Mw2000) 0.8 Example11 Polyethylene glycol (Mw2000) 0.8 Example 12 AMPHITOL 20BS 0.075Example 13 AMPHITOL 20BS 0.075 Example 14 AMPHITOL 20BS 0.075 Example 15AMPHITOL 20BS 0.075 Example 16 AMPHITOL 20BS 0.075 Example 17 AMPHITOL20HD 0.075 Example 18 AMPHITOL 20N 0.075 Example 19 AMIET 105A 0.05Example 20 EMAL 20C 0.075 Example 21 ACETAMIN 24 0.075 Example 22Pluronic L-44 0.075 Example 23 EMULGEN 430 0.075 Example 24 PELEX SS-L0.075 Example 25 Denacol Ex861 0.8 Example 26 RHEODOL TW-S120V 0.05Example 27 KF-354L 0.05 Example 28 Polyethylene glycol 0.8 (Mw2000)Example 29 AMPHITOL 20BS 0.075 Example 30 AMPHITOL 20BS 0.075 Example 31Polyethylene glycol 0.8 (Mw2000) Example 32 AMPHITOL 20BS 0.075Comparative Polyethylene glycol 0.8 Example 1 (Mw2000) ComparativePolyethylene glycol 0.8 Example 2 (Mw2000) Comparative None 0 Example 3Comparative None 0 Example 4 Comparative None 0 Example 5 ComparativePolyoxyethylene sorbitan 0.007 Example 6 monostearate ComparativePolyethylene glycol 1.0 Example 7 (Mw2000) Comparative Sodium di(2- 0.05Example 8 ethylhexyl)sulfosuccinate

TABLE 2 Poly(meth)acrylic acid Moisture Logarithmic Surface (salt)-basedparticulate CRC content D50 standard AAP Vortex GCA FGBP tensionwater-absorbing agent g/g % μm deviation δξ g/g Second g/g ×10⁻⁹ cm²mN/m EX-1 Example 1 35.1 3.3 360 0.358 28.5 21 32.6 210 65 EX-2 Example2 35.0 3.3 358 0.357 27.8 24 31.5 203 65 EX-3 Example 3 35.3 3.0 3520.356 28.1 22 32.0 202 64 EX-4 Example 4 34.8 2.7 361 0.357 27.1 24 31.0191 65 EX-5 Example 5 35.5 3.1 360 0.358 28.2 23 32.1 220 65 EX-6Example 6 35.1 3.2 363 0.361 28.4 21 32.2 217 65 EX-7 Example 7 32.0 3.1360 0.358 30.1 23 31.5 132 65 EX-8 Example 8 34.7 3.3 359 0.356 30.3 2132.0 213 65 EX-9 Example 9 35.0 2.9 362 0.362 30.5 21 32.4 195 65 EX-10Example 10 32.0 3.3 365 0.363 28.9 23 30.1 381 65 EX-11 Example 11 35.13.1 366 0.365 30.3 19 34.0 235 65 EX-12 Example 12 34.8 3.2 354 0.35630.3 19 33.9 208 64 EX-13 Example 13 35.7 3.5 361 0.357 30.7 15 35.4 20164 EX-14 Example 14 35.0 3.4 445 0.390 30.0 17 33.2 195 64 EX-15 Example15 36.2 3.1 364 0.362 29.7 14 35.1 92 64 EX-16 Example 16 34.9 3.2 3640.362 27.0 18 34.3 160 65 EX-17 Example 17 35.2 3.2 359 0.358 27.2 1834.3 156 66 EX-18 Example 18 35.1 3.2 366 0.365 27.8 17 34.5 177 66EX-19 Example 19 35.0 3.0 358 0.357 27.1 20 33.4 135 67 EX-20 Example 2034.9 3.1 358 0.357 26.6 20 33.6 175 63 EX-21 Example 21 34.7 3.3 3690.370 28.9 21 33.2 171 62 EX-22 Example 22 35.2 3.1 360 0.358 27.1 2331.9 143 64 EX-23 Example 23 35.0 3.2 362 0.360 26.9 24 32.1 178 65EX-24 Example 24 35.1 3.2 362 0.360 27.0 23 31.7 149 67 EX-25 Example 2535.1 3.4 367 0.364 26.7 22 31.5 137 68 EX-26 Example 26 35.1 3.3 3610.359 26.8 24 31.5 145 63 EX-27 Example 27 35.2 3.4 358 0.358 27.1 2331.2 135 62 EX-28 Example 28 35.1 3.1 367 0.365 30.5 27 28.5 405 66EX-29 Example 29 34.3 3.1 362 0.361 30.4 15 33.7 229 64 EX-30 Example 3040.1 3.2 362 0.360 26.2 14 36.5 38 64 EX-31 Example 31 38.2 3.1 3650.363 27.0 20 33.8 69 66 EX-32 Example 32 37.8 3.1 364 0.363 28.1 1734.5 61 65 CEX-1 Comparative 35.0 3.6 358 0.358 28.4 44 25.5 255 65Example 1 CEX-2 Comparative 35.1 3.0 361 0.362 31.5 25 34.0 15 65Example 2 CEX-3 Comparative 33.5 4.2 345 0.352 30.2 26 32.3 40 72Example 3 CEX-4 Comparative 33.8 4.1 343 0.356 25.7 24 28.3 87 72Example. 4 CEX-5 Comparative 27.0 3.5 390 0.360 27.2 32 27.0 395 72Example 5 CEX-6 Comparative 27.8 3.6 456 0.410 26.2 35 25.1 352 70Example 6 CEX-7 Comparative 34.6 3.3 450 0.312 7.2 48 23.5 86 64 Example7 CEX-8 Comparative 27.5 3.5 360 0.358 27.2 18 25.2 207 44 Example 8

TABLE 3 Weight average particle Weight diameter Polymeri- average interms Solids zation particle of dried Hydrogel CRC content rate diameterproduct particles [g/g] [wt %] [wt %] [μm] [μm] GKF1 32.9 40.2 98.3 212156 GKF2 34.1 40.3 98.2 237 175 GKF3 32.5 39.6 98.4 195 143 GKF4 33.340.5 98.3 270 200 GKF5 35.1 39.9 98.3 205 151 GKF6 36.5 40.0 98.2 203150 GKF7 32.9 40.2 98.5 212 156 GKF8 32.2 39.5 98.5 302 222 GKF9 32.539.2 97.9 311 228 GKF10 32.2 39.5 98.3 302 222 GKF11 39.6 42.7 98.2 280211 GKF12 39.2 43.8 98.2 597 453 GKF13 38.3 41.9 98.6 290 217 GKF14 34.139.8 98.2 201 148 GKF15 34.1 39.8 98.6 201 148 GKF16 31.3 40.1 98.7 234173 GKF17 32.0 39.9 98.4 211 155 GKF18 30.8 39.8 98.4 243 179 GKF19 31.440.0 98.7 238 175 GKF20 32.1 39.6 98.5 240 176 GKF21 30.9 40.5 98.5 222164 GKF22 30.2 40.2 98.5 225 166 GKF23 31.1 39.7 97.9 236 173 GKF24 31.540.1 98.1 218 161 GKF25 31.2 39.8 98.4 217 160 GKF26 31.8 40.2 98.2 223165 GKF27 30.9 40.1 98.5 222 164 GKF28 33.2 40.1 98.3 852 628 GKF29 34.139.8 98.2 201 148 GKF30 34.1 39.8 98.4 201 148 GKF31 37.3 42.7 98.2 486366 GKF32 37.7 43.6 98.2 494 374 CGKF1 32.9 40.2 98.3 945 699 CGKF5 29.149.8 97.8 367 291 CGKF6 27.2 53.5 97.9 915 740 CGKF7 — — — 5284 3916CGKF8 31.8 39.8 97.6 247 182

TABLE 4 Water- Weight average Logarithmic absorbing particle standardresin CRC diameter deviation powder [g/g] [μm] δ ξ B1 46.6 346 0.36 B247.2 346 0.36 B3 45.1 346 0.36 B4 46.9 346 0.36 B5 47.5 346 0.36 B6 46.5346 0.36 B7 46.6 346 0.36 B8 44.7 346 0.36 B9 45.6 346 0.36 B10 44.7 3460.36 B11 41.8 346 0.36 B12 46.9 346 0.36 B13 42.8 346 0.36 B14 42.3 4290.38 B15 42.3 346 0.36 B16 44.4 346 0.36 B17 44.7 346 0.36 B18 45.8 3460.36 B19 45.5 346 0.36 B20 46.2 346 0.36 B21 39.3 346 0.36 B22 46.1 3460.36 B23 47.0 346 0.36 B24 46.3 346 0.36 B25 42.9 346 0.36 B26 44.9 3460.36 B27 44.9 346 0.36 B28 45.3 346 0.36 B29 42.3 346 0.36 B30 42.3 3460.36 B31 43.9 346 0.36 B32 44.4 346 0.36 CB1 46.6 346 0.36 CB3 43.5 3440.33 CB4 43.5 344 0.33 CB5 27.0 390 0.36 CB6 32.0 450 0.40 CB8 42.3 3460.36

TABLE 5 Water-absorbing BET specific resin powder surface area (425/300)[m²/kg] B1 33.2 B9 34.0 CB1 25.9 CB5 28.8

<SEM Photograph>

FIG. 2 shows an SEM photograph of the water-absorbing resin powder ofExample 9 (particle size cut: 500/425) and the measurement conditionsare magnification of 30 and an applied voltage of 1.3 kV.

FIG. 3 shows an SEM photograph of the water-absorbing resin powder ofExample 9 (particle size cut: 500/425) and the measurement conditionsare magnification of 130 and an applied voltage of 1.3 kV.

FIG. 4 shows an SEM photograph of the water-absorbing resin powder ofComparative Example 1 (particle size cut: 500/425) and the measurementconditions are magnification of 30 and an applied voltage of 1.3 kV.

FIG. 5 shows an SEM photograph of the water-absorbing resin powder ofComparative Example 6 (particle size cut: 500/425) and the measurementconditions are magnification of 130 and an applied voltage of 1.3 kV.

[Absorbent Material Performance Evaluation]

(Evaluation Method of Physiological Saline

Solution Absorbing Speed (Core Acquisition) and Re-Wet Amount ofAbsorbent Material)

An absorbent material to be measured was produced by the followingmethod. That is, first, a water-absorbing sheet 12 (80 mm×80 mm,thickness: about 0.1 mm) was spread out in an acrylic resin container 11(inside dimension: 80 mm×80 mm, height: 4 cm), 2.4 g of awater-absorbing agent 13 was then uniformly scattered thereon, awater-absorbing sheet 14 (80 mm×80 mm, thickness: about 0.1 mm) wasspread out thereon, and a surface sheet 15 (80 mm×80 mm, thickness:about 0.1 mm) having liquid permeability and collected from acommercially available diaper was further placed thereon, therebyproducing an absorbent material 18 as a model diaper (the concentrationof the water-absorbing agent including the water-absorbing sheet: about82%).

Then, a liquid feeding apparatus 16 (weight: 80 g, a load applied to theabsorbent material: 1.25 g/cm² (0.1 kPa)), having a cylinder with adiameter of 30 mm, a height of 120 mm to which liquid can be suppliedthrough a central portion) was placed onto the absorbent material 18 asa model diapersuch that a load was uniformly applied. Next, 48 g of aphysiological saline solution (0.90% aqueous sodium chloride solution)set at 37° C. was poured in the cylinder quickly (at once). A period oftime from the time point when the physiological saline solution wasstarted to be poured until the physiological saline solution wascompletely absorbed by the absorbent material was measured and set as afirst speed (second) of Absorbing the physiological saline solution.After 30 minutes, four weights 17 (weight: 180 g) were placed around thecylinder of the liquid feeding apparatus 16 such that a load of 12.5g/cm² (1.2 kPa) was uniformly applied to the whole absorbent material,and 24 g of the physiological saline solution set at 37° C. was pouredin the cylinder quickly (at once). A period of time from the time pointwhen the physiological saline solution was started to be poured untilthe physiological saline solution was completely absorbed by theabsorbent material 18 was measured and set as a second speed (second) ofabsorbing the physiological saline solution. After 3 minutes, the liquidfeeding apparatus 16 and the weights 17 placed on the absorbent material18 were removed, a paper towel (manufacturer: Oji Nepia Co., Ltd.,kitchen towel, a product obtained by cutting into 80 mm×80 mm andoverlapping 30 sheets thereof) was placed on the absorbent material 18,and a load of 30 g/cm² (3.0 kPa) was placed thereon to be left to standfor 1 minute. The amount of liquid absorbed by the paper towel wasobtained by measuring a change in weight of the paper towel, and thisamount was regarded as the re-wet amount (g).

(Evaluation Result of Absorbent Material)

Evaluations of the absorbing speed (core acquisition) of thephysiological saline solution into the absorbent material and the re-wetamount from the absorbent material of each of the particulatewater-absorbing agents (EX-1), (CEX-2), (CEX-6), and (CEX-8) obtained inExample 1, Comparative Example 2, Comparative Example 6, and ComparativeExample 8 were measured. As for these measurement or evaluation methods,measurement was carried out according to the above-described absorbentmaterial performance evaluation. The results thereof are presented inthe following table.

As described in Example 1, in the water-absorbing material using theparticulate water-absorbing agent (EX-1) in which a balance between GCAand FGBP is highly achieved, an excellent absorbing speed is shown atthe first time and particularly at the second time, and it is alsopossible to reduce the re-wet amount.

On the other hand, as described in Comparative Example 2, in thewater-absorbing material using the particulate water-absorbing agent(CEX-2) out of the range of the invention in which GCA is high but FGBPis low, it is possible to reduce the re-wet amount, but the absorbingspeeds at the first time and particularly at the second time aredecreased so that it is not possible to achieve a performance balancebetween the re-wet amount and the absorbing speed. Further, as describedin Comparative Example 6, in the water-absorbing material using theparticulate water-absorbing agent (CEX-6) out of the range of theinvention in which GCA is low and FGBP is high, an excellent absorbingspeed is shown at the first time and particularly at the second time,but the re-wet amount is largely increased so that it is not possible toachieve a performance balance between the re-wet amount and theabsorbing speed.

Further, as described in Comparative Example 8, in the water-absorbingmaterial using the particulate water-absorbing agent (CEX-8) out of therange of the invention in which FGBP is high, but CRC and GCA are lowand the surface tension is low, an excellent absorbing speed is shown atthe first time and particularly at the second time, but the re-wetamount is largely increased so that it is not possible to achieve aperformance balance between the re-wet amount and the absorbing speed.

TABLE 6 Absorbent material Water-absorbing agent Absorption Surfacespeed [s] Re-wet CRC GCA FGBP tension First Second amount [g/g] [g/g][×10⁻⁹ cm²] mN/m time time [g] Particulate 35.1 32.6 210 65 15 65 2.3absorbing agent (EX-1) Comparative 35.1 34.0 15 65 16 86 2.7 absorbingagent (CEX-2) Comparative 27.8 25.1 352 70 13 58 9.9 absorbing agent(CEX-6) Comparative 27.5 25.2 207 44 14 62 15.8 absorbing agent (CEX-8)

REFERENCE SIGNS LIST

1 Filtration apparatus

2 Glass filter

3 Silicone tube

4 Stop cock

5 Glass tube

6 Tank

7 Support cylinder

8 High humidity strength cellulose tissue

9 Piston

10 Metal ring

11 Acrylic resin container

12 Water-absorbing sheet

13 Water-absorbing agent

14 Water-absorbing sheet

15 Surface sheet

16 Liquid feeding apparatus

17 Weight

18 Absorbent material

Incidentally, the entire contents of the prior Japanese PatentApplication No. 2015-123529 filed on Jun. 19, 2015 are incorporated inthis application by reference.

1. A poly(meth)acrylic acid (salt)-based particulate water-absorbingagent comprising poly(meth)acrylic acid (salt)-based water-absorbingresin particles as a main component, the poly(meth)acrylic acid(salt)-based particulate water-absorbing agent satisfying the following(1) to (5): (1) an absorption capacity without pressure (CRC) is 28 g/gor more; (2) GCA is 28.0 g/g or more; (3) a relation between FGBP andGCA satisfies, in a case where GCA is in a range of 28.0 g/g or more andless than 35.0 g/g, FGBP≥−10×10⁻⁹×GCA+380×10⁻⁹ cm², and in a case whereGCA is 35.0 g/g or more, FGBP≥30×10⁻⁹ cm²; (4) a weight average particlediameter (D50) of the particulate water-absorbing agent is 300 μm to 500μm; and (5) a surface tension is 60 mN/m or more.
 2. Thepoly(meth)acrylic acid (salt)-based particulate water-absorbing agentaccording to claim 1, wherein GCA is 31.0 g/g or more.
 3. Thepoly(meth)acrylic acid (salt)-based particulate water-absorbing agentaccording to claim 1, wherein one or more compounds selected from anonionic substance, an amphoteric substance, an anionic substance, and acationic substance are contained in the inside and/or the surface of thewater-absorbing agent, the nonionic substance is (a) a polyol, (b) ahydroxy group-modified product of a polyol, (c) side-chain and/orterminal polyether-modified polysiloxane, or (d) an alkylene oxideadduct of higher aliphatic amine, the amphoteric substance is (e)alkylaminobetaine or (f) alkylamine oxide, the anionic substance is (g)a sulfuric acid ester salt of a higher alcohol alkylene oxide adduct or(h) alkyl diphenyl ether disulfonate, and the cationic substance is (i)an ammonium salt.
 4. The poly(meth)acrylic acid (salt)-based particulatewater-absorbing agent according to claim 1, further comprising a liquidpermeability enhancer.
 5. A hygienic material comprising thepoly(meth)acrylic acid (salt)-based particulate water-absorbing agentaccording to claim 1.